Not enough info for what kind of vehicle.

If this is an electronically shifted transmission so the first thing I would do is pull codes out of the computer. There may be transmission codes in there even if the check engine light is off. Some shop will do that for free so get the codes and that wil help eliminate a few things. If one of the codes are maximum shift adapts you almost definitely will need to rebuild the tranny. I would not change the filter at this time if the fluid looks okay because for a car with that many miles on it sometimes it does more harm than good. Hopefully it is just something in the valve body like a shift solenoid but it sounds a little bit more serious than that.