YOu don't change any of your existing assets. LIke you said, the change is for "new" assets. So from this point forward, if you purchased any new assets in 2015 that are under $2500 ($500 if in CA) then you have the option to either depreciate, or expense them.
What I've decided for my rentals, is that if the asset does "in fact" become "a material part of" the property (such as a new central A/C blower unit which cost just under $2500) then I'll deprecate it. But if it's not "a material part of" the rental, (new range for example) then I'll expense it.
