Boot into the drive with a boot drive (if it's encrypted, hopefully you have the key and then you can use a DART disk), recover the data, then wipe and lay down a fresh OS.

There's other ways to do it, but IMO, it's best to start with a fresh OS (especially if you have to use Windows).

There used to be a program that I've used in the past called ForensiT. We had to do a large domain migration at work due to a merge-and-acquisition and we used this tool. I don't remember exactly how it worked but it helped with allowing the user log in with their new credentials but still have all their old data. I want to say it went through all the files and updated the security and permissions under the user's profile.

Depends on the data you want to save, and how friendly the apps that put it there are to the data being moved.

Assuming the data is all file-based and well behaved, and you're an administrator on the local PC the steps would be something like:

1. Create the new local account and sign in once.
2. Sign out new account and in as admin.
3. Take a copy of the new account's folder in C:\Users
4. Replace that folder's contents with (a copy of) the old account's contents.
5. Drop the new folder's data over the old, replacing any conflicts.
6. Replace the permission for all contents of the new folder.

And, of course:

*Implement a real backup plan for any actually important files. Cloud service folders are ONE copy, and don't count as backups of themselves.