Can't be done. Or at least, I don't see how.
Scenario:
You've got chrome running.
Now you want to switch, so you run eg
google-chrome --user-agent=FooBar/42
Result: In the running session, a new window
opens and eg.
http://daa.li/ua or simply about:version
show the FooBar/42 UA.
How do you now easily revert to the old, original
UA? Without closing Chrome, of course ('coz
I asked for *easily* :]).
Solution: Run
google-chrome --user-agent=$OriginalUA
Just running "google-chrome" doesn't work.