The lacquer will most likely not lift the original paint. The original enamel would have been baked on plus it has had plenty of time to fully cure. However, try a test patch somewhere inconspicuous though just to make sure. For the "real" paint job you should probably sand it back to the factory paint or bare metal anyway since any low cost lacquer or Rustoleum type coating would not be a good base for a modern paint system. If its just the roof, I'd put something quick and easy on it for now and take it back off later for the real job.

However, if its the whole body that needs work and you want to the metal protected as work proceeds, then a good epoxy primer is the best bet. This is the way I'm going to go with my 164 which will take some time to do all the rust repair and bodywork. Epoxy requires at least a cheap primer gun and an organic vapor cartridge respirator though. Stay away from the 2K urethane primers that use isocyanate hardeners these are very dangerous and require supplied air breathing systems, plus they do not protect metal as well as epoxy in this scenario.

Epoxy will seal the metal from the elements, but the sun will gradually cause it to chalk giving it the "Rat Rod" look. Supposedly, if there is adequate coverage, it will still protect the metal even after it chalks. The colors are limited to white gray, or black. Black epoxy weathers the best , followed by gray, then white. When its time for the "real" paint job, just scuff sand the epoxy, apply a sealer coat of fresh epoxy, then apply any type of paint.