I was watching the Legends of Wrestling Roundtable episode on Canada the other day on the WWE Network, and Michael Hayes mentioned on it that during Axxess leading up to WMX8, that Stephanie McMahon had been talking to fans, and that is when they first started to get the impression fans would be cheering for Hogan over the Rock. So my guess is it was probably during the last day or 2 before WM when they decided to do the turn.
I also remember watching some WWE DVD, I think it was one of the Hulk Hogan specific ones, but I cant remember for sure, but I remember Hogan was talking about the days leading up to it, and he was either in Canada at that point, or in the process of traveling there, and Hogan said Vince talked to him about wearing the Red and Yellow outfit, and as Hogan didn't bring it with him, they talked about rushing and sending Hogan all the way back home to Florida to get the Red and Yellow, and Hogan was either just about to go, or maybe even had left for Florida when Vince changed his mind again, and decide to stick with Hogan in the nWo colors for WM. Again, sorry I don't remember the exact specifics, it's been a couple years since I watched it.