Sometimes a joint implant only needs to be revised, not replaced completely. The plastic lining may be removed and replaced. Or perhaps only one side of the joint is loose. Waiting too long can lead to uneven stresses and strains on the joint. More damage than good is done in such cases.
A few guidelines may help you when deciding when to go ahead with a joint revision. Watch for:
constant pain that never goes away
pain at rest
your knee gives out from under you unexpectedly (risk for falls)
you are limited in what you can do and what you want to do
Even one of these factors can be reason enough to have a joint revision. More than one probably means you are at risk for further damage and injury, not to mention depression from inactivity. Talk to your doctor again. Find out what's needed and what to expect in recovery. It may not be as bad as you think.