The current (preliminary) upgrade guide for v7.x suggests to use v7-dev as version to upgrade to v7 (alpha), however this does not seem to work on my end. I think this is due to the v7 branch not existing, given that seems to be the branch pattern composer expects.
While one solution would be to already create a v7 branch, possibly a neater way to go would be to use a (composer) branch alias. There already is an outdated alias of master to 4.0-dev:
https://github.com/Laravel-Backpack/CRUD/blob/2d7499ed90b453444d6ab721754c045dcd48de79/composer.json#L70-L72
I think adding a dev-main: 7.x-dev (or 7.0.x) alias would probably be the easiest way forward, to easily support various ranges of constraints to refer to the new version, like just ^7.0 or 7.x-dev.