Pitfall 6: 32 bit to 64 bit. The vast majority of Server 2003 deployments were the x86 version of the OS. All versions of Server 2012 R2 are x64 only. Pitfall 5 was about hardware not being up to running Server 2012 R2. It might also be that the processors you have on the hardware only support x86 and you couldn’t install the x64 Windows Server 2012 R2 on it anyway.
Pitfall 7: You don’t know where to start. Should you deal with the easy workloads first or the ones you suspect will be more difficult? My advice is that you should move as many workloads across as quickly as possible, which means starting with the easy stuff. That way if it does take longer with the hard stuff, it won’t be holding up other workloads that are ready for migration.
Pitfall 8: Functionality that you rely upon has been deprecated. Some features in Windows Server 2003 are no longer available in Windows Server 2012 R2. Of course WINS is still available, because WINS will still be a feature of Windows Server when the sun burns out.
Pitfall 9: You rush the migration and make decisions you later regret. While at this point you are unlikely to meet the deadline, it doesn’t mean that you shouldn’t spend a good amount of time planning your migration. Measure twice, cut once.
Pitfall 10: You only realize that something won’t work after you’ve migrated. A workload you expected to work on Server 2012 R2 simply doesn’t, and rather than being able to roll back and find a solution with time to spare, you have a “Wile E Coyote” moment as you realize that things are about to go quickly pear shaped.