if the windows 7 license is oem, i think you need a new license, yes, thanks to the mainboard change, the license goes attached to the serialnumber of your mainboard
about the disc, to reinstall you can use a dvd for your exact same version borrowed from someone that has it, what matters is the license key, the 25 letters and numbers is what is unique on the license, the dvd is just a media with files
if the pc is still running press windows key and the r key, this will open the run window
there write cmd and press enter
in there write
slmgr /dlv
this will run a command that later will tell you lots of details about your license, if in no part of that small white windows says anything about retail windows, you will probably need another license for the new mainboard