quick question on this....

ive run linux for a cpl years slackware and redhat BUT ive always wiped my drive before upgradeing...

now question is i have RH 8 setup working nicely and i was thinkin about goin up to 9. can i just install over 8? or is there an upgrade option to redhat? ive never really checked nor bothered but i dont want to wipe just for RH9 so if not then ill stick with 8....