Hi Dantfo,
HG is for hyperglide, the shimano chains use special rivits that can't be reused. The new grey rivet is slightly larger so that it makes a good fit. If you use the old rivet there is a chance that it will work loose and fall out. I think that the ends of the standard rivits are slightly peened over so when you push them out the hole in the link becomes slightly enlarged. A standard rivet link remover is OK to use.
Hope this helps>
Dave.