My understanding is that if you just re-install RouterOS over your old RouterOS install, it should be okay. Do not reformat or fdisk the drive. I have not actually done this myself. Contact support@mikrotik.com for a definitive answer.
I suspect that keeping the configuration will keep the password which you do not know. Hopefully you have a text format backup of the config you can use to reload the configuration. Only you can decided if you can deal with clearing the configuration.