Incremental Compilation and SignalTap

Arria 10 or another family? For Arria 10 Standard edition, I do not believe routing is preserved. For all other families you should be able to preserve both placement and routing. Assuming Top is your only partition, set it to post-fit placement and routing. SignalTap is automatically another partition/s, you don't have to do anything special, and it will be fit around the existing design. (You should also try Processing -> Start -> Rapid Recompile if it's supported for your device, as this is meant to do a very similar thing of keeping your design locked down and fitting signaltap, but it has some extra capabilities such as handling design changes, and I believe is the "future flow" for quick SignalTap compiles.) For Arria 10, it all holds true except the design must re-route, which hopefully doesn't cause the problem to go away.