It can but does not need to, i bet epic will try to avoid any serious regretions, most fragile part is APIs which may change. You will be able to switch between version so you can upgrade version to version carefully
That should not happen. We attempt to maintain file format backwards-compatibility going back to versions at least many months before the latest one. When planned breakage occurs, we take it very seriously and work to let everyone know well in advance that it’s coming.
Historically, we haven’t attempted to maintain compatibility between versions developed years apart. For example, file formats were not maintained in moving from UE3 to UE4.
Our recommendation is that teams upgrade to the latest version fairly frequently, to keep up with the latest features, optimizations, and bug fixes. The more time passes between upgrades, the more complicated the upgrade process can be. Of course, there are exceptions, such as the final months where a large product is being completed and undergoing final testing, and the team desires to minimize code changes.