Senior Recognized Developer
Join Date:Joined: Aug 2007
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There's a good chance from a developer standpoint it'll require major rework/rebasing.
From a user standpoint, it SHOULD be feasible to do a straight upgrade without wiping. (Although in the case of features that aren't re-implemented yet when L nightlies start, they'll reset to their defaults when they make it back in.) You will NOT be able to downgrade without wiping or restoring a backup though.
As a historical reference - many of my devices that were running 4.3 Omni were upgraded straight to 4.4, even though we started from scratch with 4.4