IMO, you should always do a complete tear off and replace, same with siding. Do it right everytime and you should have few problems. Anytime you lay over (shingle on shingle) the same problems are still going to exist with the new shingles. There will be less life of the shingles because they may not all lay correctly, openning the door for more expenses. Totally agree, if there are soft spots you must replace the sheeting, ie, tear off and start from scratch. I would get a new estimate.
| 
