The 732's (ex Express) have been converted back to a mainline config with First Class. The ex-Shuttle 738's are still in Shuttle config, however they aren't being switched between mainline and Shuttle.
I guess DL might be hoping to send some of the 738's back to Shuttle if things were to improve. I've also heard that they wanted to keep the 738's in Shuttle config in order to test out how an all-coach "premium" product works on regular mainline routes. If you look at where these ex-Shuttle planes fly, its mostly on routes like ATL-SAV/MCI/HOU/TLH....routes also flown by Airtran.