What are the chances of going from a partial to a full knee replacement?

Depends: A partial knee may last 20+ years. The overall condition of your knee at the time of the partial, your age, weight, and activity level will all play a role in the lifespan of the partial.

Need a good install: If your a good candidate, then the most important factor in a long term result of a partial knee replacement is the installation accuracy. Poor result due to surgeon installation difficulty has lead most surgeons to perform total knees on everyone with arthritis. Using the makoplasty robotic system in over 500 patients I have found consistantly precise installation every time. 15-25 years in most.