Successfully upgraded to VDP 5.5 with my existing backups intact.
1-Make sure to upgrade during backup window.
2-If your platform is already upgraded to vSphere 5.5, make sure not to access 'Configuration' tab of VDP 5.1(There is a known issue. It hangs)
VDP 5.5 has some cool features. I am on my way exercising them.
By chance if you are looking for a fresh deployment, i have clear steps in my blog- https://communities.vmware.com/blogs/Backup-Buddy.com/2013/09/24/vsphere-data-protection-vdp-55-depl...
I am in process of creating steps for upgrade as well.
I was not brave enough to upgrade so I ripped and replaced since I'm only using it in a dev environment. I don't see much in the way of changes from the 5.1 version. Same poor inaccurate reporting and useless logging. The only thing of value that Avamar brings is de-duplication. We're an Avamar shop and know well the ridiculous complexity of the product... you know VMware is able to license it for free from the mothership so why not use it? Please stick a fork in it and start over vmware.
Here is a user friendly issue I've been running up against today. No useful error messages as to why it failed.
Well, just to update this topic ... I did upgrade our three VDP aplliances from 5.1 to 5.5 and it was successful ... The upgrade procedure is very slow - you should be especially patient between mounting the ISO and refreshing vdp-configure upgrade page ... Anyway, 5.5 is really nothing better then 5.1 ... the majority of the bugs are still there (for me the most annoying is VDP Backup jobs not considering new VM joined to ressources pools ) ... so if you are not planning to use the new features (I'm sure they brought new bugs) you can easily stay on 5.1.
We upgraded and have had nothing but problems since. Backup times have gone from 3-4 hours to 12 hours+ with some jobs refusing to finish.
Even when jobs do finish the appliance crashes and has to be rebooted and the web client doesn't show any successful backups. We did log a case regarding the performance and were advised to upgrade our network to 10Gb which we don't have the capability for. We did reconfigure two dedicated 1gb network cards for the backup vlan but have seen no difference in performance or reliability.
I'm now deleting it and re-installing 5.1