In general you need a Union to make any pipe fitting (PVC, metal etc) removable without a cut.
But, why do you need to remove the fitting to change the sand? THere are no access panels on the filter? I've always used DE filters on Pools, but I don't believe you change the sand on a sand filter anyway (?), only backflush.
But in any case if this is a once every 5 year thing, then make the cut when you first do it. Then put in a union at that time.