thanks for asking this question. During the planning phase of the new update capability we talked about those use cases a lot because allowing you to go easily back and force in the SR Track without touching your baseline installation is one of the main reasons behind the new way of using the SR (Service Release) Track. An update won't contain anything new which won't allow you to go back to a previous version in the SR track.
We will introduce changes to the product which are maybe not allowing you to go backwards without having a backup from your previously installed version only in the Feature Track.
Here is another link which talks about the different ways of installing the product.
Please let me know if you have any other question about this.
just to add something especially about the App Migration scenario. Since Qlik Sense 2.0 we will take care of the App Migration when updating your environment. This means as well that we are taking care of the downgrade because with Qlik Sense 2.0 and moving forward, we introduced the ability to use the App back and force in between of the same Major release without having the need of taking a manual backup of the App first.
This means an App created in 2.0.1 can be used in 2.0.2, 2.0.3, ... or in 2.1 or 2.2. But in case it is needed the App can be imported back into an environment again where a previous version is installed, even if the App has been used already with an upcoming version like 2.1. As soon as the App has been migrated once into a 3.0 format, you can go back to 2.x.x only bay taking a backup of the App and the repository first and then it will behave in the same way inside of the 3.x.x Major release until Qlik Sense 4.0 will be released.
Again, let me / us know if there are any questions.