DNS is NOT case sensitive, however it is case aware. If a registering machine's hostname is in upper case, when it registers, it will honor the case and will enter the record into DNS as such.
If the machine's host name is upper case and is a DHCP client, and you have DHCP set to register for the client (configured to use credentials), or if you don't it will honor the case as well.
If you look at the zone data as AbeerOmar suggested, to change it, during the next registration cycle (DCs every 24 hours, clients/members every 7 days) it will change back to whatever the registering machine is.
So it would be helpful if you can describe if these are DHCP clients, static clients, if using a Windows DHCP and if so, if DHCP credentials have been configured, using WINS, GlobalNamedZones, etc, to better point you in the right direction.
But as I mentioned, the basic thing to look at is that DNS is not case sensitive, but it case aware and will honor the case of the registering host.