I have dealt with stand alone ESXi hosts and trust me maintinance is a big challenge though.
However in your scenerio good thing is that you do not need to back up everything.
As you are replacing HDD from one to another and you have RAID configured. In this case what i have done in past is to replace one disk at a time and wait for RAID to do all the rebuilding work. This needs to be done very carefully. There should not be any -
-any existing faliures on hardware should be remediate first.
-Most important to ensure health of HP Smart array controller health.
In my case i had RAID-5 with 4 disks and replaced all 4 disk one by one without loosing any data. Key is to monitor RAID building process from HP iLO and wait for rebuilding to complete for recently replaced disks..