In Assassin's Creed II, after acquiring Altaïr's armor, I decided to purchase all of the other armor sets, in order to increase the villa's value. It seems, however, that I no longer have access to Altaïr's armor, and now find myself quite vulnerable. As far as I can tell, there's no way to switch between armor, so I'm left with the conclusion that it's lost. Is there anyway to get it back?
You should have access to all your armor back at Villa Monterriggioni. There's an entire room dedicated to it. :)
As you walk up to each armor piece, you should have the option to equip it. You might have to examine it first.
I believe Altair's Armor ends up there as well.