Should I ... 1. sand the old piece so that there is a groove to fit the joint compound and tape?
2. build up the joint compound so that they meet level, sand and skip taping so that it's even with the rest of the surface?
3. Build up the joint compound so that they meet level, then tape and add more joint compound, leaving a bump?
4. Tear down the new stuff and reinstall with the tapered parts removed, so that the pieces meet square edge to square edge, then do a 2 or 3?