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The slope is 2.5:12. Three roofers came out and each of them said that shingles would be a viable option. The roofer we selected then couldn't get to bare wood so he ended up replacing the plywood. He said the inspector said they had to follow the manufacturers recommendation. But if you can go over one layer of shingles with another layer, why can't you go over a layer of tar with shingles?
The reason you can go over an existing layer of asphalt shingles is that they are supposed to be flat, and the second layer can lie flat upon the first. If the surface is not flat, then the shingles will not lie flat against each other and the tar strip at the edge will not properly seal. Later a strong wind can lift the unsecured edge and tear it off. Going over an uneven tar surface will not allow the shingles to seal properly due to the unevenness.
From your last post, it seems you've gone ahead and had the roofer do it the way they suggested. I hope you have done your homework and picked a roofer who has been around for a while, since I think you will be calling him again rather soon. Then again, maybe you'll be the first who is satisfied with the results. In my area, I've got two neighbours who switched from tar-and-gravel to asphalt shingles (supposedly low-slope), put up with them for about five years each, and one has gone back to tar-and-gravel, and the other went to torch-on membrane. So good luck.