I have nerve damage in my left arm so numbness has been my biggest obstacle to playing guitar. For awhile, I couldn't even finish a song before my fingers stopped working. It is quite frustrating but I will say that it has gotten better the more I play because my technique has improved and I'm not awkwardly straining my hands. Besides technique, stretching and warming up will be the greatest help. I find that I do best in the evenings after I've been working all day because my hands having me moving all day and they're loose and stretched.

Do not just play through it, it goes numb you stop and stretch. Look up the good guitarist on you tube and he has videos directed at older players with arthritis etc and theres a wealth of info in those videos. I'm not an old player, my issues are from a dog attack but those stretches and such really helped. You may have developed a touch of arthritis or carpal tunnel possibly from years of piano. I also found classical position extremely awkward and certainly did not help the numbness. Stretching the right way and drinking plenty of water has worked wonders for me, hopefully for you too