I once played my Wurlitzer 153 on New Years Eve.  Everything was fine
except the tuning.  I had tuned the organ a few days prior to New Years
Eve and the temperature was around 40 deg. F.  New Years Eve turned
out to be around 10 deg. F. and I didn't have time to re-tune the
instrument.  The reeds sounded so bad I had to turn them off.

I don't think the cold should have a negative effect on the instrument.
All of the moving parts will get warm from their movement and since you
aren't introducing warm air into a cold machine there shouldn't be any
condensation.  As Robbie has mentioned I would tune the instrument to
as close an operating temperature as possible.

Now, as a funny side note to this story, my truck did not perform as
well as the band organ on that New Years Eve.  After having played for
four hours in 10 degree weather, I drove the truck and band organ about
1/2 mile when the truck engine died at a traffic light.

No matter what I did I could not get it started.  My brother joined me
and together we couldn't get it started.  Me, my brother and two cops
pushed the truck and trailer out of the intersection and at 1:30 a.m.
I had to call a tow truck to tow the truck and trailer home (about 5
miles).  After spending 7+ hours outside in 10 degree weather I thought
$100 for a tow was the year's best bargain!  Oh, and what was the
problem, you ask?  The fuse to the electric fuel pump had failed!

Dave Clark