So, I may not have (ever) understood exactly how Phase Shift Monsters works, so I will lay out a few things myself.

-Phase Shift Summoning requires an "A" monster and a "B" monster, and the two are used as overlays to Summon a Phase Shift Fusion monster whose level is the total of the two monsters used for the Phase Shift Summoning.
-Phase Shift monsters have a Phase A effect and a Phase B effect, as well as (usually) different ATK and DEF values for both phases. 
-Phase Shift monsters can detach the card with the opposite phase any time they want during the Main Phase (except the turn they're Summoned). Ex. Phase Shift monster is Phase A, so it detaches the "B" to move to Phase B, and vice versa.
-Should a Phase Shift monster lose both overlays, they are banished during the End Phase.
-They can be resummoned from the banished pile at any time, assuming you have the overlays necessary to Summon them.

I think that's pretty much how they worked before, but I think this explains things rather well. Right?