[nirofo]

You can ream out the hole using a 1/4" to 1/2" tapered hand reamer or similar, then tap out the hole to the new thread size using a 3/8" whitworth tap. Any good tool shop should be able to supply these tools. Be careful when reaming out the hole, don't make it too large or you won't be able to cut the thread deep enough for a secure fit to the tripod head, 5/16" diameter is plenty. If you use the 1/4" x 3/8" whitworth threaded insert make sure it is a good fit in the newly tapped hole. If you cannot get the reamer to go in far enough due to proximity to the lens, then you'll have to revert to drilling the old thread out, make sure you obtain the correct drill size for a 3/8" whitworth tap, I think it's 5/16". Make sure you cover all the various parts of the lens to prevent metal swarf from entering, tape it if necessary.

nirofo.