the gist of it is 20-I-5 can only be used if the reassignment is to cover an inbound crew being late or due to their schedule needing repair. Key word inbound. Most reassignments in my experience happen after a loss of flying, so 20-I-5 or 20-I-9 wouldn’t apply.

in my situation both crews (us and them) were on a long tarmac delay and the company brought us both back to the gate early (before the 180 mins and them before us) we had slightly more remaining block (cco) than they did so they gave us their pairing to cover since it was a long leg. However, since that flight was the trip’s originating leg (hence crew not inbound) you can’t apply 20-I-5. I figured something was fishy so I pdr’d it and lo and behold. The crazy hard thing was getting the crew desk/scheduling to recode the pairing with ALL the subsequent legs which are then eligible for the 125% percent add pay, which only then can pilot crew pay adjust the register.
I would pdr all 20-I-5s. And make sure the pairing you are covering is referenced in the comments, so you can do some digging. If you didn’t lose flying, there is a chance you may be entitled to more.