I have 2 routerboards and can not connect to them because username and password is unknown. Is there a way to reset it without loosing the entire configuration?

No.

No.....

How can one go about connecting to RB750 r2 when the password is unknown. Taking over IT support and ex IT left no documentation. Resetting config is NOT option. Anyway to log in?

Look at it like this: You're some guy who doesn't know the password and wants to get into router. If you can do it, what prevents some other random guy who doesn't know the password from doing the same? So the short answer is "no".

The long answer is that there's some chance. If you happen to have unencrypted backup from router, you can read the password using program from http://manio.skyboo.net/mikrotik/. If you don't have such backup, you can find some inspiration on same page and attempt to use network boot to get into router. But considering the difficulty of doing that (there's nothing prepared for RB750), resetting config might start to look a little less impossible.

Look at it like this: You're some guy who doesn't know the password and wants to get into router. If you can do it, what prevents some other random guy who doesn't know the password from doing the same? So the short answer is "no".

The long answer is that there's some chance. If you happen to have unencrypted backup from router, you can read the password using program from http://manio.skyboo.net/mikrotik/. If you don't have such backup, you can find some inspiration on same page and attempt to use network boot to get into router. But considering the difficulty of doing that (there's nothing prepared for RB750), resetting config might start to look a little less impossible.

This only workable on old version RouterOS and password length not more than 12 character.

I can't comment on network booting part, I've never tested that myself. But as long as backup is not encrypted (/system backup save name=<name> dont-encrypt=yes), mtpass still works (tested with RouterOS 6.39rc55):

Can the password be reset by using Netinstall where is checked the "Keep Old Config" and install all packages from same router version?

if u do not have a master pass, u can "see" it in settings.cfg.viw file, under winbox appdata folder