D2 Auto slipping

As title says on a slight incline/hill the vehicle sets off ok but when starting to climb the revs go up but the vehicle does not increase in speed. But if I kick it down it changes down and pulls off and then changes up, it seems ok on the flat The oil level was a bit low but no change after topping up. This has come on all of a sudden, it did not do it one day then the next time out it did. I have seen another thread that says it could be a clutch pack slipping, could this be my problem?.

change all the oil and filter in it before getting scared IMO

Could be, but not necessarily as the changing points are controled electronically depending on engine load, road speed throttle position etc. So there may be an issue there, OK the box went into kick down ratio when "flooring" the throttle but what about a simple down change.. Is that okay.
Download a copy of RAVE that you list some test that you can do yourself along with others that you can't, unless u have the tools.
Assume u have a diesel, but the D1 diesels, on steep hills the vehicle won't go up any faster but the gearbox may change down and the so the engine revs increase but that steep hills and not a slight incline.