Quick question on roof repair.

strip all repair deck replace all the shingles.

do it once do it right then go relax..........

Just reshingle what you have to remove as you state. Why completely reoof unless necessary?

On a small shed you should be able to manage a complete tear-off. That is how to do the job right especially since the shed is small.

It can be done, but it won't be easy or likely pretty. When you have removed enough to fix what is broken, try to slip #30 felt under the old (easier if the old is #30), tack it down, start from the bottom as if doing a new roof, work up to the old, slipping under between the felt and shingles, you may have to gently wonder-bar a few nails out of the old or cut them off with side cutters, you will have to drive nails from the top to lock down the final course, and tar the heads (hint: tree pruning spray)

You could TRY sliding new shingles under the edge of the old ones, but if they're anywhere near the end of their life, they'll just crumble and you'll end up replacing them anyway.

best reason so it all matches.

say the existing shingles are 1/2 thru their life......

in not too many years you will be replacing shingles that arent that old.

go to sell your home and it will be a big hassle.