Language, form, muscle memory (call it what you will) is difficult to separate from thinking and working. I'm very picky when it comes to desktop UI: I use Linux exclusively, and I can't tolerate most Linux distros' default desktop environments. Someone who's been productive for a decade or more with Windows applications -- well, to the extent we're willing to ascribe "UI stability" to those applications' own updates -- will probably hate Linux with a passion.
I don't think such a transition can be made seamless. They should have thought about becoming Microsoft's hostage two decades ago (I guess).
When you migrate anyway you could choose that to use a proper database and SQL if that makes sense instead.
Yes, there is a cost to changing software. But it’s not unique to an Open Source migration.
Being stuck in legacy systems sucks, and technical people like to deny the reality of it - but it's a business reality.