hey
can i use hacks from ubb 6.2.1 on ubb 6.3.0 classic?

Some of them work, but check for the hacks in the 6.3 forums first. If it's not there use the 6.2 ones. Lots have been ported from 6.2 - 6.3.

No matter what you do be sure to back up every file you plan to modify, also back up the members directory incase the hack modifies them in a way that you can't easily reverse.