According the leaker JediPaxis, there were at least 3 different ways the film opened at one point or another.

The first opening I recall was what you described, it would start with an extended flashback of Luke and Leia training, then it would flash forward to present day, with Leia training Rey in the same place.

The second opening he described started on Exegol, where we would see the communication tower activate, and the Star Destroyers rise out of the ice. Basically this change was made because they felt the stakes needed to be introduced at the beginning of the film. I imagine this could’ve been just propelled by music, but I also could picture Palpatine’s broadcast being heard at this point too.

Then the third is what we got, which starts with Kylo on Mustafar.