Nothing happens with the already uploaded files.
You can also create a custom validator instead of having a lot of content types, see IValidate here http://world.episerver.com/Documentation/Items/Developers-Guide/EPiServer-CMS/7/Validation/Validation/
Had a similar problem a few months ago and solved it by using image resizer which both convert to jpg and scales image on the fly (with excellent disc cashing for performance).
Then your editors can upload whatever but you will still have great performance front end...
This solution worked great for me and took an hour or so to implement (and some hours more to change some image urls)
Might be an alternative which requires no editor work...
I have a generic ImageFile content type that uses MediaDescriptor for jpeg, gif, png, svg etc.
Now I have a requirement to restrict the type of file a particular content reference can take to only jpeg. (To stop people from using massive PNGs or BMPs for a certain property).
Is there any reason why I shouldn't just create a contenttype for each file type so that I can use the AllowedTypes attribute accordingly? So I'd have something like this:
I guess I'm wondering what effect that would have on existing images that were uploaded as an ImageFile originally.