G'day all.

I've dug up a flash disk with an ancient install of Mikrotik 2.9rc1 on it. No-one here can remember the password from back then. I don't suppose I can do a Net Install or something on it to reset the password and regain the use of the license?

Yes, reinstalling will reset config, but license should stay. I don't know about this old version though, I think you will need to purchase a new license to use v3. You need to find an installer from back then

Ooh, found a newer CF card, 2.9.43 - Same problem, tho, lost password we've gone through so many.

How do I go about it? Just drop the CF card into my laptop and tell NetInstall to install?

yes, netinstall with unchecked "save config" will do it

I've plugged the CF card in, loaded up NetInstall, clicked on the drive, and nothing really has happened, all the software-id/key/save config fields are disabled.

Ah, that's sorted.

Another question, What are the upgrade rules? Can I take this 2.9.51 to 3.18?

Boot it up and check under /System License to find out what you can upgrade to.
-Louis