You have to re-tape. Tip- if you are connecting tapered edges of new
sheetrock to cut/ untapered edges, taper the old edges by crushing them
a little with a hammer/ mallet. This way, when you add mud and tape,
you will be filling a gap, rather than building a bulge. You will be
doing that anyway with extra coats, but easier to feather it out if you
don't overdo it at the outset.