Isn't it just a matter of using a technique with a name like "taking ownership of files/folders" or something like that ?
If I remember correctly, you do that from the properties window of the main folder, and it should go relatively fast.
Once done, you would simply have to "reinstall" MSFS, choosing that exact folder as destination. The install program should be able to see the files in there and understand that it may not need to re-download everything.
But it's just a direction to investigate for you. I hope more knowledgeable members will give more accurate details.
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