I'm afraid, there is not a direct function to achieve this. Well, if all leaf nodes have no computer evaluations you could start backsolving again (with computer evaluation) and CPT would reset all other computer evaluations.
Another approach could be to export your whole sub-repertoire as PGN (maybe it is necessary to do so without position evaluations) and then re-import the PGN file into a newly created sub-repertoire.
OK, ok... I'm convinced there should be a better option 
Regards,
Stefan