Yes, it happens. Even on very expensive cards. One of the guys on the PSA message board just increased a Jim Brown rookie from an 8 to 8.5:
https://forums.collectors.com/discus...m-brown-rookie
I had a Gold Classic Jeter /9500 that I sent in that got a 9. Sold it and forgot to remove it from my inventory. Noticed that when I spotted it next, it had been upgraded to a 10.
Some of the 1993 SP Jeters that are now 10s were once 9s.