ezo
Hero
O' contraire! It works very well IMO.It never works.

We already had a homebrew that does precisely this: Warlock is a sublcass of Cleric, and Sorcerer is a subclass of Wizard. Warlocks retain Invocations, and Sorcerers retain Metamagic. Also, most of the subclasses for each were moved to other classes where they also worked very well. If you want me to dig through the homebrew for those I can get them to you later today probably.
If you have any questions about how these worked in actual game play, let me know.