A free answer I’m not sure of… but there are relatively cheap programs like Photoshop Elements which contain batch rendering that can take an entire folder of pics and re-configure things like resolution.
As for media off line and search… if you change the name of the file then the media will for sure go off line and the search feature won’t work because it’s looking for a different name. If you change the pic type (change the extension) then that will constitute a name change and you will have to redirect the media off line search manually with each pic. I *think* though that Excalibur has a “media replace” feature if I’m not mistaken?? It’s free to check out anyway.