Reverse actors HP MP progression

JSR and JMP are identical instructions, except JSR allows you to easily return to the point you jumped from (with an RTS instruction). Since you're interrupting an existing routine for a bit of custom code (and not writing a brand new, custom subroutine), you'll definitely want to RTS afterward.

For example, here's a section of the block the Edrin pasted above:

Code:

C0/A286:    8D0342      STA $4203      (save as a multiplier)
C0/A289:    B90816      LDA $1608,Y    (character level)
C0/A28C:    851B        STA $1B        (save it for now)
C0/A28E:    641F        STZ $1F        (zero out upper HP byte)
C0/A290:    AE1642      LDX $4216
C0/A293:    BFA07CED    LDA $ED7CA0,X  (Initial HP)
C0/A297:    851E        STA $1E        (save initial HP)
C0/A299:    A600        LDX $00        (X = #$0000)

Wherever you interrupt this routine, you're going to want to finish executing it after your custom subroutine has run, which is where the RTS instruction would come into play.