Exchange 2007 Move-Mailbox help

HI,

I will be migrating uses from one E2K3 forest to a E2K7 forest.  My steps will be first to use ADMT, to migrate the user. Then use the Move-mailbox command.  For this I am not sure if I should use the -allowmerge or -NTAccountOU option.  I tested this earlier and the -NTAccountOU created a account during the move-mailbox step.  This was an issue because I already had the account when using the ADMT.  Once this is completed I will use GalSync.

Please let me know which options would work best.

Thanks