If 1 replace my outdoor unit, does my indoor unit need to be replaced too?

Ideally you want to replace both units at the same time; AC unit and outdoor heat pumps are built to work with a matching indoor unit, when you mix different kinds of units between the outside and inside this will lead to inefficient performance and overall output. By getting a new outdoor and indoor unit at the same time you will get a warranty covering the whole system so you will not need to cover any repair costs during this warranty period.