From my own experience with the baby GM's and I say this because they are manufactured at the same factory as the 071's, a cocktail stick and some T-Cut, white spirit on a cotton bud (q tip), a curved scalpel blade and gently scrape towards the printed number, be careful though as you won't want to eat into the paint finish and cause scratches. I have tried them all, and had good results. The curved blade takes time and a steady hand as I have said. The white spirit will leave a glossy effect after the numbers have been removed and this ideal for when you apply your new decals.. The T-Cut and cocktail stick takes time and patience but you get there in the end. One bit of advice I will say is to use Microsol on the area where the decal is to be applied, and brush on some Microset after the decal has dried, it will help remove the carrier film from the decal. Make sure that you use some kitchen towel to remove any water from beneath the decal or you could get air being trapped beneath it. You can then give the finished model a touch of varnish to seal the decal in.
If you have an old model have a go and see what suits you, just be careful and you will be fine. With the arrival of DCC and DCC sound it seems as though the renumbering of models has taken a back seat in how we change our models from out of the box items, but it does give the model it's own uniqueness. Have a go and let us know how you get on.
Best of luck,
Rich,