The DB was overhauled at MCD 4.0 (rel 10) to bring remote directory synchronization onboard and eliminate deep dance on Opsman. For single sites this is not much of an impact other than the fact that the upgrade must be staged first to MCD 4, then a database migration to convert the DB to the new format, then upgrade to MCD 5.
at each stage, the DB is incompatible with the next so it is imperitive to get backups before and after each stage.
In the background, the licenses on the Mitel AMC are also changed to a new format that cannot be rolled back.
If you were my customer and a standalone system I would discuss if your plans might include a 2nd system in the future as the license types are different at Rel 5 than at 9. If yes, I would recommend purchasing the networking option (if you don't have it already) prior to upgrading to convert your licenses to Enterprise instead of standalone.
There is probably a half dozen things I'm not mentioning but that is sufficient I think to convey the complexity
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What's most important is that you realise ... There is no spoon.