What we know
- Kitty and Professor Xavier will play a factor in the time travel method.
- Wolverines mind will travel back into his younger selfs body.
The problem with the time-travel that i can see is that in pretty much all other media the events of the past occur in real time and the events in the future are instantly changed due to the events in the past changing.
No matter which way they do it they'll be telling the stories in both eras ('1973' with Logan, and the First Class cast and 'The Future' with the original trilogy cast) in real time, that's gonna prove difficult.
For an example, if Wolverine in the past changed something then logically it would cause a ripple effect that changes the events of the future instantly. But since the future as it is is part of the flowing story and not subject to change right away.
So how do we think they can tell a flowing story with both eras without Logans meddling in the past causing a ripple effect to instantly alter the events of the future and allow the future part of the story to continue throughout the movie?
Perhaps Xavier and Kitty while linked to Logans mind are outside of time with the events of their time changing around them. This would however have the rest of the OT cast change around them.