Question about Seller Financing involving an existing mortgage

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Hi fellow investors.  I have a question for those of you who may know the answer.  I have done many seller financed deals in the past where the existing property was already owned "Free and clear" by the seller.  The deals worked out great because I simply make my mortgage payments via a loan servicer and everything is kosher.  This past week, however, I found some seller financed deals where the seller had an existing mortgage on the properties and wanted to involve a land contract in the deal for 3 years.  The homes would be titled to the seller for 3 years and then titled to me after I refinance the note somehow.  During closing though a new mortgage would be created, down payments made, and an interest rate established.  It's not all 100% clear to me how these deals are structured as ultimately you have to ensure the seller pays their lender with your monthly payments.  It almost sounds like a lease w/ option to purchase deal, but with a few caveats.  I've read that things can get dicey because the seller's lender ultimately holds the 1st position lien and can foreclose on the properties should the original seller decide not to make his payments.  Then your investment gets kiboshed.

What are everyone's thoughts?  Has anyone done deals like this successfully and is there a way to structure it so that the buyer can be protected from foreclosure or penalties due to the seller not paying his/her note to the original lender?

I've seen more than one example of this type of deal too.  Several in fact so it seems like it's being done - just don't know how successfully.

Thank you for any feedback!