Example: Normal rend damage is 1d3+1, based off of claws (I'm aware these are not secondary attacks, just humor me).
The +1 is from the assumption of a +1 Str mod.
If the claws are then made secondary for whatever reason and their damage reduced to 1d3, does the rend also become 1d3?
Rend is, per the rules, "usually" based on claw damage. Like many things in pathfinder's monsters details, that's not an always thing.
So, it depends. You're likely correct in assuming so, but exceptions wouldn't be surprising.