When the smaller stems became popular, it was common to use a grommet. It cut down on our tube inventory. We didn't have to stock both stem sizes. We would just give the customer an adapter if he had the 5/8" hole. There is nothing wrong with using the grommets. We weren't concerned on how they looked in those days. 5/8" stems that you can vulcanize on can still be bought. We used to vulcanize stems on all the time if someone either hit the stem with a tire hammer or if someone mounted a tube off center and it rubbed a hole in the stem. I wouldn't recommend cutting a small stem off in order to vulcanize a 5/8" stem on. I'm not sure how well today's tubes vulcanize.