Reconditioning is the process of bringing a piano back to good playing order by only doing the necessary replacement, cleaning, repair, and regulation on that piano. It doesn’t involve the replacement of major components. Rather than being restored to new, it is work done to improve the pianos performance in its present condition, keeping cost in mind.
Rebuilding involves completely disassembling the piano, doing all of the necessary repairs and replacing damaged and worn parts. This includes all major components, and also usually includes refinishing the case and bench. When the piano is rebuilt it has been restored to original condition or better.