1999 E300 turbodiesel

[zakkkk]

One time recently my son was driving our 210 turbodiesel sedan and the trans would not upshift.
It stayed in 3rd and would not upshift. He pulled over and turned the car off then restarted and all was fine. My son said it hasn't happened again. I test drove the car this morning and the trans shifted perfectly. ( BTW, I had forgotten how beautifully that cars drives).
The car has 138K miles and has been trouble free except for the sad onset of rust spots in the usual places. I want to keep it forever.
Questions:
1. Does the no upshift symptom point to any specific problem in the trans ? 2. Is there a way I can check the trans fluid level and color? There seems to be no dipstick.
3. Can I change the trans fluid myself or is it a dealer only job? My MB dealer quoted 450 bucks to change the fluid/filter. Last time it was changed was at about 85K miles.
Any insight would be appreciated.
thanks
jz