Should I do a clean reinstall of old OFP before installing OFP: Resistance?

This is absolutely unnecessary. It will make no difference to OFP: Resistance's own installation.

However, some people report that OFP: Resistance encounters problems when you try playing OFP: Resistance (not when installing) if they leave their old addon files in OFP's ADDONS subdirectories.

So, after installing OFP: Resistance, if you try running it and encounter problems such as system freezes, crashes to desktop (CTDs), extremely slow performance or other anomalies, try moving all unofficial weapon, vehicle, soldier and island addon files out of OFP's subdirectories and restart OFP: Resistance to see if removing the unofficial addons resolves the problem.

If you have some other reason to reinstall old OFP, do it before installing OFP: Resistance. Here is my recommended reinstall procedure.