I believe a recorded quit-claim and documents indicating a second deed of trust are necessary. The buyer of the 2nd will want to be sure there is clear title and no liens other than the two mortgages.It is also very important to notify the holder of the first mortgage of the change and request notification if property owner is late or in default on payments. This is so owner of 2nd may make the payments on 1st and protect his interest while foreclosing
It might be a good idea to talk to the bank and also try to pick the brains of a broker who deals in 2nd's
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