Seems like a harmless suggestion. Still, I would also recommend only 1 per character for the life of that character. Granted, I'm not sure why I feel this should be how it's done, but it just seems right.
As for all your enhancements being sold back automatically, why not just have them auto converted to the appropriate origins anyway, since they would all cost the same.
Only exception would be Yin Talismen, since each origin gets a different type of SO there. In that case, they either go buh-bye, or they get sold back as originally suggested.
I agree with willow that it sounds pretty harmless. I'm not sure the devs would go for it though. They seem to be really against letting people change the most basic things about their character (AT, powersets, height/gender up until very recently) and this seems like it might fall along the same vein.
Four things define a character to the devs:
AT
Primary powerset
Secondary powerset
Origin
Not height, not gender, etc.
Of them, Origin "does" the least, as far as we know. Yes, in Alpha, it did a lot more. But it's not, on the other hand, just "window dressing." While many people use IOs, just suddenly switching origin would render DOs and TOs "incorrect." We don't know what sort of effect this would have on the character. Similarly, blueside at least, starting contacts are tied to origin (Kheldians excluded.) We don't know what sort of effect having an "invalid" contact would be.
They do also have effects on some temp powers - including which power is awarded, or extra effect. This could just be a check on the power, or there could be a separate power awarded when an origin that matches the bonus is noted. (I dont' *know* this, and doubt it's a separate power, but the possibility is there.)
Epics, of course, are tied *to* their origin. It's part of their backstory. They'd have to be excluded. *shrug*
I'm not against the idea, mind you - just pointing out that it *could* have more of an impact than first glance.
Writing code to switch innapropriate enhancements on an origin switch would not be impossible.
Failing that, allowing Origin switch on a respec could also work. It makes in-game sense - a previously Magical hero has an "incident" at Terra Volta and becomes a Science hero with modified powers - well as much game sense as the nucelar reactor/Respec Trial thing ever did 
.
I've got a couple of characters who I hadn't quite concepted out before I started playing them, and would like to switch origins on them. Not enough to really keep me awake at night, but it would be nice to be able to do so.
So /signed.
How about a origin change token? Just in case you accidently hit the wrong origin or say you thought up a really great story to go with your char but the origin doesn't work, thats where this would come in handy. And since they don't work, you can sell all of your privious enhancements. I say 1 awarded to each char on account would be fair right?