subscr_009.s (switching), subscr_091.s (force out), subscr_114.s (baton pass), and subscr_175.s (pivot attacks) all deal with switching. All of these need to be updated to perform the HP restoration after the switch, but before showing the party list.
Documentation
https://bulbapedia.bulbagarden.net/wiki/Regenerator_(Ability)
Relevant Code Points
subscr_009.s
(switching),subscr_091.s
(force out),subscr_114.s
(baton pass), andsubscr_175.s
(pivot attacks) all deal with switching. All of these need to be updated to perform the HP restoration after the switch, but before showing the party list.