Slightly more specific, you need to upgrade the bootloader and firmware on the E2T cards when going from anything from before 9.0 to 9.0 or later. I usually do all of my PBX upgrades myself (not a Mitel VAR, just a sysadmin) and our VAR strongly recommended that we either have them come out for the upgrade, or at least keep them informed of when we were upgrading because it is a much worse upgrade than usual.
I'm having trouble finding the email chain back and forth with my VAR from when we made the hop, but I do recall that there was an entire separate section on upgrading the bootloader when upgrading from a pre 9.0 release, and in our specific instance I had to set a static IP for the E2T on all three of our PBX's because they weren't picking up DHCP after the upgrade.
On the bright side, once you get on to 9.0 or later, upgrades become ridiculously easy.