Transformation to and from Hellbat Mode now requires the Transformation Servos upgrade from the Factory Tech Lab.
Transformation Servos requires an Armory and costs 150/150, with a 110 second research time.
 Developer Thoughts
David Kim has posted on the Battle.net forums about the next Balance Update. He addressed issues in ZvT with Hellion contains followed up by Hellbat aggression once the Terrans Armory was completed, stating the likely change will be to bring back the Hellion to Hellbat research requirement, with it being in the Factory tech lab and require an armory, to give Zerg players more time to scout and prepare against Hellbat based attacks.
"Now that the highest skilled pro players have been playing the beta for a few weeks, we've been gathering pro level ladder data, pro feedback, as well as watching all the HotS tournaments/show matches that have been going on.
Out of everything we're seeing and hearing from pro players, we think the most important issue we need to deal with rather quickly is early game ZvT.
The biggest issue in the ZvT early game at the highest pro level seems to be the mass Hellion contain leading into the various potential mass Hellbat combinations right when the Armory comes into play. The time between Terran gaining full map control and then converting all their map control units into straight up power units looks to be too narrow.
Even without this threat, Terran has plenty of early game strategies to utilize vs. Zerg. We're most likely going to add the Hellion to Hellbat transformation upgrade back to the Factory Tech lab with an Armory requirement. This will give Zerg more time to scout and prep against possible Hellbat based attacks when they see lots of Hellions in play."
So now before suggesting the upgrade be removed. Try to think about what that does to the ZvT early game.