I have been using Debian Lenny 64 bit on xenserver 5.6 for some time now without any issues.
It's worth noting that i didn't migrate a 64 bit machine from 5.5 but setup a new machine.
In my experience a cheap testing environment is a really good thing to have when testing out new releases like your scenario.
The ideal is to have a identical testing environment as your production enviroment but in most cases this isen't a viable option because of the cost.
As a good practice when migrating to newer solutions you should do a isolated test run.
I would setup a isolated xenserver 5.6 with local storage only and create a snapshot from your production debian server that you export to the testing environment.
This way you could try it out without risking downtime on your production system.