Just cut the existing ones off as close to the top as possible, dig out the wet foam, re-foam, re-glass...and tab them to the hull as needed. The ones you saw totally removed were the all plywood '66 models, or the Prolines (for the most part).
Fuelman, It is a little more work, but it is really worth the extra effort in my opinion, as there isn't a whole lot of glass in these old hulls.
... we re-glassed the inside and outside of the stringers.
Quote from: "gran398"... we re-glassed the inside and outside of the stringers.That must have been a pain for the person grinding.