Three things I have to say about this:
1) I have a sorcerer; I love playing her.
2) I would absolutely love this feat; it would allow me to tear apart literally everything, with little to no investment and effort. If this feat were to be put in game, I would, without hesitation, pick up this feat (and, likely it's successor, Energy Adamixture)
3) I do not want to see this feat (or any other feat like it) in game.
In the end, this would allow casters (sorcerers, in particular) to be grossly overpowered. Imagine having all four "wall of (element)"'s on your list, so that you can simply use one spell to kill (literally) everything. What point would there be to even having other spells on your list, when two or three spells would suffice for your entire damage stock?
No, I think energy substitution should lie in the hands of the Savants (the whispered about sorcerer PrE), but, if it's done as PnP is, that would substitute every energy type with one specific energy type. Which would be okay, so long as they gave ways to pierce DR, but... thats another topic.
In the end, good suggestion with good intentions, but I see this feat being universe shattering, more than game breaking.