Switching armor coating is something we can't do yet within the game.
However, it would be possible to achieve the ice armor artificially by converting parts of its body directly into ice like skin or to give it a ice like xenoskeleton set of armour.
As to balooning itself to become large, while we can't nessecarily do that, we could try a unique sweat/hostility trigger secretion that dispels a syndrome carrying liquid that can artifically modify its overall size (getting it to shrink back down again might be difficult unless the syndrome is short lived on activation or is dispelled by the creatures immune system)
Yes. We can even make it spit globs of deadly cold (i know a bit about mh myself, though im a 1st and second generation kind of mh player after never really getting into 3 onwards) as a attack, in its larger size is probable that it will become more rather than less effective but, the important thing to recognize that you'll have to make a small amount of compromise on how it turns out but mostly we can recreate it well.
Tip - [SECRETION] tags are commonly on the core humanoid entities (dwarves, elves etc) to handle extreme emotion and exertion, however you may be able to get somewhere with interaction:material_emissions too if spit is too small/direct for you to use.
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Spit molten rock]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:MOUTH]
[CDI:MATERIAL:INORGANIC:BASALT:LIQUID_GLOB]
[CDI:VERB:spit a glob of molten rock:spits a glob of molten rock:NA]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:30]
A big depository for tokens to help you can be found here
DFwiki - Tokens, and more specifically for breath/glob related interactions you can find the reference here
DFwiki2014:interaction tokenYou know what vision you have for it, and i've detailed the tools to help you, i wish you luck and offer more guidance if you need it rather than step in and do it for you (doing it yourself aids the process of teaching yourself the fundamentals, making you a better raw modder in general).