The Apple Authorized Service Providers are a coveted & profitable closed loop. My Dr. Macbook made service personal.I have never heard that before that be heat paste needs to be replaced every 3 years. But they all seem to be running quite well and cool, and all are well over three years of age.
If you need the serial numbers changed to match the original, Apple provides their authorized service dealers with a utility to do just that if your Fix-It guy has access to Apple's service resources.
I don't think I've ever sold a Mac, just kept on buying new ones! Aren't Macs supposed to last forever???