Have done it for a number of customers of ours. You basically backup the old servers - the application backups and if you can do old server image backup using backup softwares like Acronis that will be much better. If you don't make sure you have copy of all the 6.x softwares (SP,CM ISO template,SM Kickstart/software,SMGR ISO, etc.) in case you need to revert back. It is the revert back process that is the most critical one as you are going to wipe-out all the 6.x.
Process would be:
1. Backup all 6.x systems
2. Download softwares you need 6.x in you revert back and 7.x OVAs, patches, SM/SMGR data migration utility
3. Deploy 7.x systems VMs (AVP or VMware)
4. Run data migration
5. PLDS licensing rehost
Hope this helps.
jtc22