As described here, you will at least be able to change the default uid for the next session and onward. The following is what makes it happen in short:
"You will need to end all connections to that computer, for example with the command "net use \\[server] /d", before you can create a connection with the other set of credentials."
"With the command "net use \\[server] /user:[username] [password]" you can specify a specific set of credentials to use for the connection."
BTW: Make a big fat note about fekking windows drive letters. If you have any assigned to the same server as you whish to invoke the commands above at, the commands would seem to work (especially under command.com), but they wont. A special pit-fall is if you've assigned a drive letter to a server identified but a IP number and not a name - the old credentials will be used no matter how you try until that "drive" is disconnected (grr.. may all Windows lovers burn in h3ll...). IMHO, better avoid drive letter alltogeather. It was a bad idea since birth and a nuicense.
Anywho...:
To handle changing credentials smoothly, you can create a script for Cygwin like this:
#!/bin/bash
net use '\\aserver' '/d'
if [ $# -lt 2 ]; then
echo "Connecting to default user"
net use '\\aserver' '/user:adomain\auser' 'apassword'
else
echo "Connecting to specific user $1:$2"
net use '\\aserver' '/user:adomain\'$1 $2
fi;
