metal over old roof

Seems first attept at posting failed. Trying a second time. Sorry if the first one went through and I didn't see it. We're replacing a 22 yo shingle roof w Aluma(?), 24 g metal roof. The installer has sstated that only the shingles need to be removed, not the felt between the shingles and the plywood underneath the shingles. What we have, then, is the 2X4 that form the main frame, the 4X8 plywood between these 2X4, then the original shingle felt, over which a plastic-like felt was installed, then the metal roof itself. I may not have explained this very well, My apologies. So, what is the correct thing to do: remove or not remove the original shingle felt? Thanks.