If you stepped into the same issue (cannot transfer ownership as of Workspaces account), no worries. The migration can still happen, and even Google Sheets documents with embedded forms can be migrated.
What you need is to share the file, and then make a copy. In case of Google Sheets document with embedded form, it would create a copy of the embedded form as well.
It is also possible to share a folder, but things get tricky with that. Although you can sometimes move files out of the folder to your own drive, the original owner still remains the owner of the document. So a copy has to be created to become owner.