Again, this can be subject to change so I can’t be definitive on this but from my understanding based on the Blizzcon panels, you can just freely travel to do what you want. The choice to level the old fashioned way is not taken away, just reserved for those that wish to do it that way.
The only stipulation they mentioned was that new players would have the new starting zone, then go right into the new expansion. Once that’s been done once (again, the specifics not nailed down), the option to level the time traveling way is open.
Lastly, just because I can’t stress this enough, this is all subject to change so feel free to speculate and provide feedback on how you would like it to work, but keep in mind nothing is set in stone just yet. 