Some pictures would help and what type of Material is it? My personal rule for flats even if they don’t leak is tear it all off. If it leaks already there is moisture in it and you cannot go over it with another layor.
Moisture will become trapped between the new and the old and can cause a multitude of problems including the deck continuing to rot, blisters, and not adhearing properly.
If you want the new roof to last anywhere near normal life life span do yourself a favor and tear it all off.