That is the very reason folk are getting annoyed at Microsoft for. Its no good making documents that only you can read. (unless of course you are the only one who sees it).
Its a ploy by Bill to force upgrading?? to the new format. I have used MS Office since 1990, but have now moved to OO.
To go back to the original question, there is nothing to physically swapping the MS Office suite to the new puter, its been done before and will probably be done again.
As to the legality, only you would know, and would be for the user to worry about.