Hello Everyone:)May I ask for assistance on possible transmission issue?Thank you!

[jetairliner]

Hello everyone. I am new to this forum. I have no mechanical education or experience So I would like to ask for your experience and advise. And I apologize if my question has been answered somewhere on this forum already. I have a bit hard time finding it. But any way, the question is about my E39 '96 (523 IA euro specs.) Steptronic. (about 163.000 km.)
I have an issue with gears some times not switching in automatic from 4 to 5 until I reach speed of 130 km. per hour. RPM's of course go up pretty hi, up to 5 k. And I am a bit worried about transmission, since car did not use to behave in this manner. Yes, 5th. gear is for cruising speeds, and for the weight of this car it should reach at least 80-90 km.\hr. in 4th. gear until switching to 5th. But 130 km.\hr.? and why does it switch exactly at 130km\hr. now? I had full diagnostics done and there were no errors reported, so electronics seem to be fine. But, as I've mentioned before, transmission did not behave in this manner before.. I would say it started a few month ago. some time after I've changed all the spark plugs, filters, including fuel filter. Could this be the result of it? I doubt it though.. If anything car should perform better... Any way, these are just my thoughts that are going through my head about this issue, I apologies for the chaos And hope for your more logical and coherent input What could cause this issue? Maybe I need to change transmission fluid? Or just add a bit? I understand BMW recommends not change transmission oil for these vehicles. But they recommend a lot of things just to get your money in the long run. But if it is done, (transmission oil change) there is very strict procedure. It has to be done in different transmission temperatures with very precise amount of oil added in several steps. I was also recommended by bmw specialized mechanic to simply add a bit of ATF oil up to filling shaft. But if I was to simply add oil my self, how would I know how much needs to be added if the procedure so strict?
I am grateful for your time and assistance with this issue!
Thank you very much.
Best regards,
Aleks.