It will update the file when you attempt to open it. I have been using Pages for well over a decade and have never had an issue with the current version not opening an older version. I would suspect that a newer document is more likely to not open in an older version of the application than the other way around.
Also keep in mind that as features change, it has to accommodate those changes. So like say a particular font may no longer be available so it substitutes an equivalent. Things like that. Been pretty seamless.
I started with Pages when it was running on Power PCs... now we're all the way to an ARM version. That's three massive operating system platform shifts.