1998 Passat AutoTrans problem (stays in gear too long)

Hi there,
as above the issue is the car stays in each gear longer than normal before changing up and sometimes changes down with low revs.
The transmission is a 'sealed' unit so I cannot simply check the fluid level as a mechanic suggested it sounds like the fluid may be low hence the lack of pressure to change up - any thoughts or am I up for a transmission rebuild?
(note, the car powers on in the set gear until it finally changes)
Thanks again