I am new to the forum and a new B250 owner!
I had a 1997 VW Jetta that was new from the dealership when i was younger. The Jetta also had a transmission that was sealed for life as per the dealership and engineering. The dealer told me that it never needed to be changed as the fluid was a full synthetic.
Post warranty i went to a Mr Lube location for a quick oil change, and was pressured in to a transmission fluid change. I explained what the dealer told me, as well as showing them the manual. I was told that they have never ever heard of that. I was then told if i didn't change my transmission fluid i faced a catastrophic failure.
Being young and impressionable, i followed their advice. I had the fluid changed.
Two days later on my way back from Whistler Mountain to Vancouver at 02:00 am, my transmission went "BANG!" and the rpm gauge dropped to zero. The accelerator pedal did nothing.
The car coasted to a stop. I got out in pure darkness on the old sea to sky highway to see a trail of transmission fluid behind me. I was stuck for 2 hours waiting for a tow truck.
A good friend recommended a close friend of his who is a transmission specialist and he found a transmission for me and installed it for a reasonable price.
Now, the lesson learned was that the reason it is sealed for life is that it was engineered that way. It has magnets built in to collect any chipped gear teeth and hold them in place... The flushing/change dislodged those pieces and they got caught.
Unless there is a TSB or the manual calls for a change... I would be leery of changing the fluid as it may lead to unknown issues.
Just my $0.02, and best of luck
However, if Mercedes Europe says a change is necessary, they superceed North America!