Something I have used is called ELO (Easy-Lift-Off). Got it from Micro-Mark.... Think a 8 oz can is about $10.00... It last for ever or it seems to. Think I have had this can a few years now. I just scrub the area with a old toothbrush... takes a couple of times, but the decal comes off clean... It will remove old paint too and doesn't seem to harm the plastic.
Ken, it depends on which manufacturer you're working with. The lettering and numbers are generally not applied with decals but painted. LGB lettering can generally be removed with denatured alcohol. Bachmann paint is more difficult. The lettering and numbers on Aristo Craft and USA are pad painted and most difficult to remove as is Hartland. I generally paint the entire car when working with these manufacturers.
Changing to your own road name is very rewarding and adds a personal touch to your railroad.
Stan, I also have some aristo 100 ton coal hopper cars and would like to just remove the last number and add different numbers. Mine are all numbered the same. Can that be done and can you make me new numbers to apply to replace the one removed. I have the Union Pacific road name cars.