Modus automatic transmission

Hello - new here and would be glad of advice. My 2006 Modus Dynamique 16V Auto, Petrol, 55K miles, has been a bit sluggish recently. By that I mean that it doesn't seem to be in the correct gear and accelerate in the same way as it used to. Especially when I set off in the morning and try to get up to speed on the faster road - I just have to wait for it to gradually gain speed and then cruise. Putting my foot down has little effect these days. I am concerned that this will get worse and I will have trouble accelerating fin various situations, which could be dangerous. There have been no warning lights showing, but my local garage says the computer has been showing several faults, which they don't understand. They just recommend getting rid of it. I am wondering if it is worth draining and changing the transmission fluid ( (Renaultmatic D3Syn or D2 are recommended in the handbook) or even just topping it up. Never been done to it I think. It may be a 'sealed for life' transmission, not sure what it is...maybe Jatco JF404E 4 speed compact or JF009ECVT. does anyone know how I can find out? But I believe 'sealed for life' can be topped up/changed anyway. I'm reluctant to spend too much on it because of its age - it also has a dent and a loose indicator cover which I have not dealt with. But it is a very practical with high seating position. Any comments greatly appreciated.