Is it possible to fix the case checking in /etc/passwd?

Question

linux

Hello!
Configured on ubuntu samba+proftp.
Entered into the domain, authentication through windows AD.
Of course, I could be wrong, but it seemed to me that the login did not depend on the case. Renamed and reintroduced into the domain - now depends on the login case. If it is in /etc/passwd, then it takes it from there. If we change the case - then already from AD, and substitutes another home directory. He is what is important to me.
Question - is it possible to somehow disable the case check in /etc/passwd? What would the login from /etc/passwd be used for in any case?