I have BA XT 4Sp Auto semi but gear changes between 2nd 3rd can be rough under light acceleration and slip under heavy acceleration. Its now done 123450.00km but had filter and oil change not long ago as, it was doing it back then so at 90,000km it was done. Does auto trans fluid become weaker as more km put on oil. I never owned an auto before this car, but understand how they work. Im thinking could be some band adjustment maybe or clutch worn ?
Much thanks if any has any info.

try about 300ml of trans fluid into the gearbox. that usually fixes most minor slippng probs

Clutchpack possibly.