I have to do some repairs on a garden shed. I need to take off about 5
rows of shingles and replace
one of the roof boards on the bottom edge.

Now, when I put new shingles on, should I:
1) Re-shingle over the entire roof, meaning that the bottom rows go on
the new wood, and then I cover the remaining shingles as I move up the
roof, which will leave a bump.

or

2) Just re-shingle up to the remaining good shingles, and push the new
ones under the old ones.

Since it's a small roof, the cost and time for shingling are not an
issue. But I don't want to rip up any more old shingles.

TIA,
Jim