ADMT across forest

Hi,

I have encountered some issue on migration across forest to Exchange 2013:

1) How do I set the ADMT migrated account use new specific domain name, a.new.com instead b,new.com?

2) ADMT account first Outlook start up after start up very slow, but not subsequent Outlook launch.
    Is this normal like for 20-30 mins on first start up Outlook launch?