Marines Mos openings in October?

Situation: My recruiter told me there were little openings in the 3500 or 0300 field which are fields I only were interested in. Since the only way for me to go into DEP for now was to sign open contract, so I did. My recruiter told me that before I ship out to boot (OCT 11) that jobs I wanted would be open and I could go back to meps, and change my open contract status to a different contract that will lock on the fields and get a better guarantee rather than the open contract. Is that true or is he screwing with me?