I came across a tool that I think is useful for removing old caulking. I found it in a salvage store and it appears to be British. I had trouble getting on the .UK website but did find this. I gave a dollar for it so I have nothing to lose.
Remove Caulk and Silicone (http://www.installerstore.com/CornerTape-Caulk-and-Silicone-Removal-Tools.html)
I have made little use of it yet so don't have a lot of experience with it.
Gary B