A couple of months ago KAXT LPTV station serving San Jose transitioned to DTV. They put in place a efficient and cost effective system that can serve as a model architecture to any LPTV station that is planning a migration. The unique thing about KAXT is the use of 6Mhz (19.39 Mbps) single channel to squeeze in 12-20 digital TV and radio sub-channels. KAXT employs efficient MPEG-2 VBR encoders and statistical multiplexer to achieving this amazing throughput.
An attempt to show how the actual deployment looks like.
The Playout servers can still spit out Analog Video. What that means is you don't have to upgrade them for the transition.