Preventing this switching (IMO prevent it completely - ie, different saves. Or at least add some other balancing feature.
The balancing feature is already planned, and it's designed around preventing mode switch in combat. There is a full discussion in the design doc archive about this. I think the main idea was to have the ship float without a pilot for a brief time, so it could still be destroyed after the player logged out of the game, or tried to log into a different mode.
Isn't that enough? If a player chooses to be in All Online and gets into trouble, they can't escape by switching modes.
This doesn't impact the ability to earn Cr's in Solo Online and then switch to All Online with a better ship. But nobody in Solo Online is getting any P2P combat practice, like they would be in All Online. For me, that's the other way this is balanced. Someone who grinds up to an Anaconda in Solo Online is still a green pilot if they switch to All Online, and they aren't going to keep that ship for long.