During his rundown of the announced card for WWE Day 1, Dave Meltzer noted in the Wrestling Observer Newsletter that there are several ways this storyline can play out and it seems like they are setting up for Heyman to reveal that his firing was a ruse.
Last night on SmackDown, Heyman never said that he was back with Lesnar and instead he put over Reigns as the greatest Universal Champion of all time. The way things are playing out, it looks like Lesnar's claims that Heyman is his advocate is just a game to get in Reigns' head and in reality, Heyman is still in cahoots with Reigns.
Meltzer wrote in the Wrestling Observer Newsletter: "There are different ways they can go, including a Lesnar title win to set up a WrestleMania rematch, or Lesnar on the verge of winning and it comes out that the Heyman firing was a ruse and he double-crosses Lesnar. The idea should be what is the best way to build WrestleMania. It also could be that a Heyman double-cross of Lesnar could be saved for WrestleMania."
Whatever happens at Day 1, it needs to be something strong enough to keep fans interested so they can do the blow-off match at WrestleMania. It seems unlikely that they would want Heyman off TV for months so the most likely scenario is that Heyman returns at the pay-per-view and the swerve happens.
As it was noted on WrestlingNews.co earlier this week, it looks like Reigns vs. Drew McIntyre will be happening after Day 1 so it would make sense for Reigns to retain his title.