I just moved a 3750 switch from a stack where it was #4. I wiped out the config by holding in the mode button, then checked it through the console port.
Then I installed it in another stack as the #3 switch, removing the stack cables while the other 2 switches were powered up and connecting them to the unpowered switch #3.
I then powered up the #3 switch. Everthing seemed to work fine except that the switch shows up as #4 instead of three. Did I miss something here?
If you manually change the stack member number by using the switch current-stack-member-number renumber new-stack-member-number global configuration command, the new number goes into effect after that stack member resets (or after you use the reload slot stack-member-numberprivileged EXEC command) and only if that number is not already assigned to any other members in the stack.
Here is the link for the bible of 3750 STACk :-
You can renumber the switch. See the Add a Provisioned Switch to a Switch Stack paragraph. The link should answer your other questions as well. For even more detailed info, check the configuration guide for the 3750s.