Went back and looked at your original post. There are certainly a lot of advantages to keeping it 243, if you can eliminate the risk of accidentally putting a factory round in it. if you have no other 243 rifles YOUR keeping things safe will be pretty easy. If you have other 243s then going the other way could get exciting and I do not mean in a good way!
If the rifle turns out to have real accuracy potential 243 match dies are easy, probably off the shelf easy. Starting load data, every where. With your GI brass you will have to account for reduced capacity from the wall thickness plus the reforming to smaller dimensions.
You have been thinking about this for a while, you have more of the pluses and minuses in mind than anyone else, putting them all down on paper should make it easier to decide.