There shouldn't be any downtime when migrating domains between registrars. All you have to do is initiate the transfers from the destination registrar. Usually when you do this, it'll let you put in your desired nameservers or the new registrar will just use the current nameservers. This way, once the transfer goes through from the source registrar's end, the destination registrar will take over without any interruption in service.
Namecheap also has this page which should allow you to do bulk transfers.
Note: You may need to obtain an EPP code from your current registrar and/or unlock the domain(s). Here are Namecheap's directions for GoDaddy transfers.