There is no need to replace the new _sersup.llb with the old llb.
Just open your application in LV 7.X, and the serial vi's should work properly.
You need to have the VISA runtime Engine installed on the target machine, for the new serial functionality to work.
If you are distributing executables, then during the build process, select under
Installer settings----->Adavnced------->serial port suppoprt.
If you do that, the VISA runtime engine need not be installed on the target machine, and the installer will just install the necssary files to enable VISA serial communication