We do have software assurance. We had Bell in for a quote and they will only support an upgrade to version 8 (their words) but we cannot go that far with digital sets. 6-SP3 is still a supported version with digital sets so I don't understand either! There are only two companies local who could do the work and the other said the'd do it but their quote wording scares me "best efforts"; makes me think they do not know what they are doing. I looked after older Nortel BCM systems for years and have upgraded those, I'm sure this is a similar process, just need software!
I don't know the process for BCMs, but the process for upgrading the MCD is multi-fold... you need the 3300 software on an active FTP server and you need the installer's root level password, then you need to run the software installer, it logs in, backs up the system database, re-licenses the controller, installs the software, performs a restore and license sync, and brings the system back into service.
Not trying to scare you, but I have seen this fail more than once for a multitude of reasons. Then you have to do the manual method, which involves connecting to the serial port on the front of the controller and changing the boot parameters to boot from FTP and start the install script, then you have to update AMC, license, and restore the backups all manually.
Your dealer which holds your license record should be more than willing to perform this service for you, if they are not willing to I would contact your regional office from
http://www.mitel.com/contact-mitel and complain and try to get them to put some pressure on your dealer, or take over the account and have Mitel Direct handle it for you.