The cost of an upgrade is dependant on several factors:
1) - The software assurance status licensing with Mitel
2) - Other applications that might be impacted or need upgrades as well.
3) - The number of sites (Networked Controllers) that require upgrading.
If you have 1 controller and if the software assurance is up to date and if you have no peripheral applications then the cost should be minimal. (Labour only, if your service provider is honest)
That is a lot of ifs.
Can you do it yourself introduces some more ifs such as if your willing to accept the risk that if something goes wrong you might need a professional to fix it.
suffice it to say I don't know enough about your system, skillset, or environment to properly answer your question.
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Occam's Razor - All things being equal, the simplest solution is the right one.