I've got a couple of old, around 20 years,  Shimano Bantam Curados, a 200B5 and 100B.  It appears that replacing the bearings with "high performance" bearings is fairly common with these reels.  Other than the original bearings wearing out is there any other reason to do this.  Both reels still seem to work as well as they did new.

Those reels came with high quality bearings. I’d flush them and spin test. If they’re quiet and spin well oil them lightly and keep using them. 

Sounds reasonable, thanks.

I have the original bearings in both of my 14 year old Curado 100 B's and they are still singing along.  I do keep them cleaned and oiled properly.  Never felt the need to change them.