Thermal paste question

It is never recommended to reuse thermal paste, you can do it but it may suffer some for some performance. It is best if you clean it off with some 90%+ Isopropyl alcohol/a lint free microfiber cloth, once cleaned off completely from both the CPU/Heat sink, then reapply it following the directions from the manufacturer of the thermal paste.

Yes. When removing the cpu heatsink the thin film of thermal compound on the cpu and heatsink becomes very uneven. In addition if you try to reinstall without removing the old thermal compound air bubbles may be come trapped between the heatsink and cpu. I recommend you remove the old thermal compound, clean the heatsink and cpu, and apply new thermal compound.