So if you "accept" a clearance and then figure out you don't like it, what happens when you try and change it? Does, in real life, ATC refuse your request to change or do they really insist that you, for example, climb into known icing conditions in your uncertified airplane?

Sometimes, and sometimes.

In general ATC works with the pilot, especially if the pilot isn't being
a hardnose. I've re-negotiated clearances and gotten what I wanted -
for example not to fly out over the sea. Sometimes I haven't gotten
what I =wanted= (direct JFK direct CMK) but gotten something I could
work with (roundabout via Sparta). And sometimes I've just been stuck
(freezing layer 6000, filed 4000, MEA 3000, but the only clearance we'll
get in the NY area is climb maintain 7000, negotiate in the air. It was
a broken layer and I ended up taking the clearance after an hour of back
and forth on the ground where ATC would not budge. I dodged the ice up
to 7000, then got into cloud and ATC would not give me lower. They were
willing to climb me to 9000 where I was on top.

I guess that counts as ATC helping to fix something (I got 9000 and
clear of clouds) but, while the airspace is crowded in the NYC area, I
should be able to get a 4000 enroute altitude in a non-deiced cherokee.

Jose
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