You can get the previous versions from
Sourceforge,. I forgot to mention that the RHEL5 version wasn't available for 4.5.5 but I did have the the RHEL4 version running on CentOS5 initially and that ran OK.
[edit]Another option is for you to install the new CentOS5 + Zimbra 4.5.10 and do an IMAPSYNC for the mail, csv exports for the contacts and REST export/import for calendar data. Really depends on the data you're trying to move and what planning you've done in advance.