-Came up on assignment to Fort Drum.
-In reenlistment window.
-Would like to change assignment. Possible?
-Already reenlisted once for 1 year stabilization/school option at current post. Would like to continue here.
-If not avail, can I reclass?

Since you are on assignment instructions (and a mid-career Soldier), you cannot reenlist for Option 3 (Army Training a.k.a. reclass while in reenlistment window)) or Options 4 or 5 (OCONUS and CONUS stations of choice). You can only reenlist for Option 1 (Regular Army a.k.a. "needs of the Army") to meet the service remaining requirement (SRR) for your new duty station/unit.