The significant difference is that they are spirits of fire, and physical fire damaging them as a primary cause of their transformation is less satisfactory than their own fire blazing with hate being the primary cause. They should turn into balrogs before the Lamps fall, or during the attack.
I was concerned that with the Lamps being consecrated, the fire that poured out would be
holy fire, or rather holy light, like that of the Silmarils themselves, which might generate great heat, but like the burning bush of Moses, would never burn up on its own - the Lamps were intended to shine for ages, after all. regardless of any existing affinity with fire, I would expect this to cause problems for servants of Melkor - to obliterate their bodies, at least, the way Beren's own hand was withered and dissolved by a Silmaril...
but there could be an interesting way to rationalize all of this. if Melkor himself assisted in the construction of the pillars, it could be that he was still considered one of the Valar, and that despite his evil, the
consecration of the Lamps might not have affected him, or those of his servants, or the creatures of Arda in general, who were immortal... he was able to avoid detection in Valinor, after all. if, like Melkor, the Balrogs were not already cursed at this stage (if their existence was unknown, for instance), then and only then does it seem feasible that they could survive destruction by holy fire, to whatever extent they might survive destruction by fire in general (bearing in mind that Sauron's own body was destroyed in the drowning of Numenor... and yet his spirit survived, as theirs could also have done). their relation to Arien is interesting in this respect (in her capacity to withstand the heat of the Sun), and I wonder if the Lamps being mythical might hint at a connection to the conflagration caused by Melkor's assault on her... but despite initially thinking that the divine light of the Lamps and the dark fire of the Balrogs could not possibly be one and the same (imagining that their fire was their very life-force, and that they were evil before ever entering Arda, or possibly from their very conception), it could just as easily be a curse in itself, a form of everlasting torment - was this the idea? everything from the shadow of wings once possessed, to the ceaseless burning of their bodies could indeed be the effect of exposure to consecrated light, for a being that is evil, yet still immune to death...
but it raises the question of what exactly is it that hallowed light reacts to, if not the corruption of Melkor? and who would be more sensitive to it than Melkor, and the servants of Melkor? while it matters, maybe, how the casters of such a spell perceive their enemy at the time of casting, the purpose of hallowing the light is obviously to protect it from
anyone who would try to attack it, regardless of who they are... but it could be that they mistook the nature of Melkor's corruption of Arda early on, and this attempt at defense was akin to treating the symptoms rather than the cause.
I was wondering myself if the light of the Lamps affected all creatures equally (when spilled) - if some might not be considered holy, perhaps in connection with various Maiar/Ainur... the Balrogs would seem to fall into that category.
it's hard to say how far back Melkor's deception goes, whether to the beginning of Arda, or much further still... but I like the idea that the Balrogs are fallen Ainur, and I imagine their fixation with chaos would be reflected to some extent in their physical forms as far back as their conversion, or their creation. if it's not until after the Lamps that they take on a more bestial aspect, maybe this could be seen previously in crests, or the design of their weapons and armor... perhaps the corrupting influence of Melkor could be seen over time, as their designs tend more and more toward decadence and morbidity.
the notion that Balrogs could have been previously angelic is fascinating, regardless of how far back that past may stretch. I love that their shadow wings/cloaks might be a kind of vestige of wings once possessed, although if they were the creations or offspring of Melkor, wings of darkness (and hearts of fire) might be symbolic also, of their vain ambition to rise to heaven and finally seize the light, enveloping it with darkness, even as the divine light within them has been captured, stifled, and corrupted... that their shapes (and that of their "wings") might be inventions, in other words, which reflect the designs and obsessions of Morgoth.
the note about there being seven makes for a tempting comparison to the seven princes of hell... but I think any number could be justified, if the term "balrog" only refers to a magically potent leader of the demon kind (I'm sure the number 24 has an interesting rationale). this interpretation could justify the earlier accounts of there being hundreds or a thousand Balrogs or more - an entire race - if only that sense of the word is assumed by something like Umaiar. these would be less fearsome, but still very frightening enemies compared to orcs, who can be frightening in their own right...
a proper Balrog could be an arch-demon, basically, as opposed to a "regular" one - if that interests.
as for the construction of the pillars, does ice still play a role? that the pillars formed seas in their collapse potentially implies it still, but I imagine the explicit reference was removed due to the apparent foolishness in allowing the pillars to be built from a material that could potentially melt - although perhaps the Valar were not as yet fully aware of the properties of ice, doubtful as that seems. maybe they were aware, but were assured that it would not melt on its own? yet this could have been achieved by heat transfer through steel cores, which would have been an even more novel material than ice, surely... what do we know about the attack? or was there one after all? I favor the idea of some kind of built-in sabotage, even if a hidden property of matter seems hard for a Vala to overlook, but steel seems like a more likely candidate than ice for this, because it can only be produced artificially. might even tie in with faeries' abhorrence of iron, although one could attribute this to practically anything.
the possibility of everything prior to Sun and Moon being mythical (more so even than Third Age events, which despite the style of the narrative sometimes strike me as still not being fully literal yet) has me
deeply intrigued, though... is anyone else considering what that could mean, if it were true? or what Elrond would make of it? as there is more to the Valar than they are able or willing to share with Elves, might there be more to Elves than they are able or willing to share with mortals?
I'm particularly intrigued by any possibility that the Lamps have some magical or symbolic significance beyond the simple provision of light... any connection to Melkor, Balrogs, Sauron or Numenor could be telling, somehow.