I havent actually renumbered any stock - but there is a great article in the British "Model Rail" No 105 - June 2007. The article refers to 2 different methods: 1) Metho & cotton buds 2) Scraping the old decal with a knife.
For the metho-method:
1) Rub area with a cotton bud soaked in metho - changing the bud frequently as it becomes soiled. Dont press too hard or the base colour will also be removed.
2) Immediately rinse the surface with clean water to remove any remaining metho and then rub the local area with 1000 grade wet/dry paper, used wet, taking care not to remove any raised rivet detail.
3) Ultra-fine grade steel wool (grade 00000) can then follow to remove any remaining marks, rubbing in a circular pattern. again, take care not to remove any of the base paint.
4) The area can then be buffed with a clean toothbrush to restore an even sheen. if any white deposits appear then traces of spirit are remaining (if so repeat steps 2 and 3)
5) Spray a lite coat of gloss varnish to give the new transfers a suitable surface to adhere to
The knife method:
1) use a very sharp scapel/exacto to scratch the bulk of the old transfers off
2) clean up with metho and cotton bud
Hope this helps!