There won’t be any upgrade path from 1.x to 2.0. v2.0 is a ground-up rewrite on a different platform, so there’s no such thing as running a Composer update to get from 1.x to the new 2.0 beta.
There will however be functionality for exporting and importing apps and service configurations from 1.9.x to 2.0. This won’t be in the public beta of v2.0, but it will be an eventual feature of the new version.