The only reason I'm even mentioning this here is because it's rather silly for this assembly to throw out the bulk mod you used on it.
Because you could've just used the bulk mod to make that red armor last twice as long. Which seems more worthwhile unless I for some reason have a much better armor lying around.
Were B-modded red armour to keep its high durability when modded into fireproof, it would by the same token have to keep its low durability if you modded it while damaged. I love being able to slap a T-mod on my B-modded-but-heavily-damaged red armour to restore its durability, and I'm not sure I want to lose this.
Also, bear in mind that fireproof red armour will last longer than regular red armour because of how durability works (armour loses durability equal to hitpoints lost from an attack).
But yeah, I also see where you're coming from, I typically (if possible) like to have red nanofiber as my main armour, and switch to fireproof for certain circumstances (mancubus complex, Tower of Babel, etc.).