I actually got this working and it was very straight forward, let me try and explain:
I removed the hard drives from the Dell T20 and moved them to the HP Microserver (it was pretty much a bare install anyway, apart from some IP camera recordings which I was happy to start again with). I then modified the grub.cfg with the updated NIC's and plugged it in to the Microserver expecting to reinstall. When I went to the synology assistant it prompted me to log in and all was well.
As I had previously set everything up in DSM 6.0 I had basic RAID groups assigned to each hard drive with a volume with each. As I removed 1 hard drive and added a new one, I just removed the old RAID group and created a new one.
Everything seems fine, very straight forward.