OK to replace one power tube for amp?

Tubes age just like people. As they get older, they slow down and are less dynamic; they don’t have the same sparkle and don’t have the same frequency extremes. They are just like an aging athlete; they can still play the game, put not at the same level.

The best way to determine if you need new tubes is to buy new tubes and compare for yourself. If there is no sound difference, put the new tubes away until that day comes.

Short answer, no problem. If the tubes are getting close to their life expectancy, then you may consider changing out both tubes for improved sound performance.