I think I have once done that upgrade once in lab and it just worked. Make sure you have backups.
Please make sure you get upgraded from ClearPass 6.4 to 6.6 or better 6.7. Depending on the amount of config, you could consider spinning up a new ClearPass on the new ESXi version, then use backup-restore to get the config ported over.
If you are in doubt, please contact Aruba TAC to review your upgrade plan.