say I've got three computers, and I put mirc on them, and register with the same key on all three.

then I want to replace one of those computers; and intend to remove mIRC from the one being replaced.

am I going to have to buy another license key for the replacement machine, even though the number of machines using that key is not, in theory, exceeding three, the maximum number of machines a key may be used on?

I would surmise that this could also apply to having to format a machine's HDD and reinstall everything.