What if retargetting was instead done this way:
Start casting (subject to interrupting). When casting ends a character holds a charge of this spell. Not subject to interrupts now. Character portrait should indicate somehow, that spell is ready. If you start casting another spell (or use whatever else ability you have) the charge is lost. You switch to the character, chose your target and released the spell. Time, spent between steps 2 and 3 is subtracted from recovery time (to a minimum of 0).
At least it could be useful for spells with long casting times that target something other that self, I think.
(At the very least it was my impression it’s not how it works currently from the way people are posting about it)