This is what PE wrote on the matter of rehalogenisation for re-using halide materials for first stage exposure:
Originally Posted by Photo Engineer
This can be done, but it causes the film to lose all of its original sensitivity due to the destruction or disruption of the grains and the original chemical and spectral sensitization placed on the grains by the manufacturer. The film emulsion reverts to a simple blue sensitive emulsion with low speed, usually in the ISO 3 - 40 range if you are lucky. If it is high enough in iodide, you may actually fog the emulsion totally.
This subject comes up from time to time. It is a common minconception.
Also, no rehal bleach should be attempted with a color film or paper bleach. The ammonia content is enough to dissolve a fair portion of the finer grains rendering the film or paper useless. Any rehal process must be conducted in an environment free of any silver halide solvent.