replacing loco gears

[leon]

As always in the past few years, Doctor Wayne, you give good advice for one not to be afraid to follow. What is concerning me is the gear set. It consists of a worm, ideler and an axel gear. If the set has the same number of teeth I would change only the idler gear. But if the teeth are different what problems would I possibly be up against when replacing all three gears?

And in reply to Nachoman: this locomotive is HO guage.

Leon