Yes moving the fish will move the ich, but ya can do a hypo salinity treatment and rid the tank of it in the 45, just don't move any inverts over. and you'll need to QT EVERYTHING for 6 weeks before adding to the tank, inverts, algae etc.
BUT here's what ya need to do more. Research your fish! do you mean emperor angel? That requires a HUGE tank, and a 45g is not nearly enough for a volitan lion IMO
, they get over a foot long! IMO
a tank this size is just gonna keep the fish stressed and have ICH issues till it dies!