To answer your water change question: the general recommendation for a 1 gallon is 1x 50% change AND 1 x 100% per week, so both of the suggestions you've heard are partially correct. :)
I think it may have been the 4-5 degree temperature change that stressed him so much, combined with all the moving. If his immune system was already compromised in some way (by poor petshop conditions or some other cause), then all that may just have been a bit much for the poor little dude.
I hope he recovers soon! :) Wrapping the bowl in a towel to give him a bit of darkness (though as Myates says, don't make it completely black - leave one side uncovered to give him some light) may help destressing him.