Signatures needed in the FUTURE for a 'subject-to' deal

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Hello, if I purchase a subject-to property from someone, cure the default on the mortgage, and keep making payments to hold the property long-term, for what reasons may the original seller need to come back to the table to sign off on documents 1,2,5 years do the road?  

I'm concerned that the original seller may be hard to find, and/or not interested to come back and sign off 'down the road.'

What steps can I take to deal with that problem/make it a non-issue NOW?

Thank you!