Could you not just use the hulls for static AA duties in ports and such? Leave them some ability to move under their own steam.
If you want a cheap refit to make then into AAA destroyers, why go through the trouble of tearing out machinery? Why not skip that step and leave it there? They can do static AAA duty with the ability to make 30 knots just as well as they can if you strip them to 10 knots- but leaving the boilers in place would be cheaper.