There are no plans to allow running on previous versions at this time.
The sim uses a number of cloud services that are synchronized with the sim (not just scenery data services, but also auth, profile, marketplace entitlements, settings storage, etc) and the team does not have plans to maintain the multiple versions of the large amount of backend infrastructure that would be required or take each service and abstract in into a cacheable model.
We understand sometimes that updates can cause unintended issues, but the team strongly believes that having the full community (which will now also include XBox) on the same version ensures the best possible feedback and telemetry to the team to get those problems sorted quicker, and avoids the user confusion of having multiple versions of the product running side by side.
Matt | Working Title