Graham,
I am afraid that the long answer is no or it depends.
There seems to be a challenge in this:
"I have a doc with about 250 images embedded in low res and need to replace them to hi-res. Anyway I can do this automatically as opposed to going on eack image in the links panel and pressing relink and finding its hi-res version."
If embedded, you have to start over with each.
If linked you may be able to replace each by removing the low res versions from the source folder and insert each corresponding high res one at each prompt, without going through the links panel.