Hello, I solved it so I'll leave the 'solution' in case anyone encounters the same issue:
You'll have the Process Instruction (PI) where the user enters the value of the parameter that defines which PI to activate.
Create another PI with the following elements:
Long text (for title)
Control Command: PROC_INSTR.DEACTIVATE, EVENT: DOCUMENT.GENERATED
Control Command: PROC_INSTR.ACTIVATE, FORMULA: (PARAMETER UPPER PI) = X (EXPECTED VALUE)
Cheers.