I can’t speak for all future versions, because it depends on what changes go into them. However for ClassicPress v2.x this shouldn’t be necessary.
So far, the only known way that something might break when we move to version 2 is if a plugin or theme requires some part of ClassicPress that we’ve moved out into a core plugin, but we don’t know about that dependency and that plugin gets disabled or removed. This could lead to plugins trying to call functions that don’t exist or don’t do anything any more, for example.
In practice that could mean some plugins will need to add a line like
“Depends on: ClassicPress/core-xmlrpc”
to their docblock in order to continue working with ClassicPress v2. I think we’ll also be able to fill in these dependencies ourselves for at least the more popular plugins.