Might have to check your contract. But I know at my store it doesn't. Unless it's been for more than 6 weeks or so I believe. Hence they try and encourage peeps to start bumping some of the part-timers after a while if need be.
I cant comment on your situation with out knowing what your union contract says. However here in Atlanta (local 1996), if you are full-time you just have to work a 40 hour week every 8 weeks to remain full-time. If you wanted , you could work 32 hours every other week.