How to switch main and backup servers while retaining data

[vale.maio2]

Hi all, I've got a bit of a conundrum, which is more difficult to explain than it actually is. I have currently 2 servers:

• Main server: Dell R710 running PrmoxMox 7.4-3, 8x 4TB drives in RAID5, total available space of 28TB. Running 2 VMs, one of which I couldn't care less; main VM has around 10TB of data.
• Backup server: HP ProLiant DL380p, 8x 1TB drives in RAID5, total available space of 7TB. It's running the latest version of PBS (can't remember the version number).

Now, I'd like to switch servers, so that the HP becomes the main server and the Dell becomes the backup one; to do so I'd have to also swap the drives, as 7TB on the HP server are not enough for my main VM. Of course I can't just swap the drives from one server to the other and call it a day, as they use their own RAID system and won't be able to read each other's data. I might have enough external hard drives to backup the main VM on them (3x 4TB USB HDDs), but I'd have no idea how they would come into play.

Could anyone smarter than me give me any suggestions? How can I switch servers without losing all of my data?

EDIT: forgot to mention, these are my personal home servers, so downtime is not an issue even if it's for a few days.