An adapter is provided for converting an irrigation sprinkler from an open case impact drive rotor to a closed case drive rotor, without requiring disinterment or replacement of a sprinkler case having the drive rotor housed therein. The conversion adapter comprises a generally cylindrical adapter sleeve for installation into the sprinkler case following lift-out removal of the open case drive rotor upon disengagement of a lock member such as a snap ring. The exterior surface geometry of the adapter sleeve is shaped to define a gap between the adapter sleeve and the top of the sprinkler case sufficient to permit re-engagement of the lock member to retain the adapter sleeve within the sprinkler case. The interior surface geometry of the adapter sleeve is shaped to receive and support the closed case drive rotor which is retained therein by an additional lock member such as a snap ring. An annular cap plate is mounted at the top of the sprinkler case to bridge and close the gap between the adapter sleeve and the sprinkler case.