In the great khan quest "oh my papa" i have everyone convinced but regis, i have karls notes but i can not get regis to agree to switch to the ncr. What am i missing?

You have to talk to Papa Khan when He, Regis and Karl are sitting at the table together, you'll get an option in dialogue to show the note to Papa.

Once that fun is over talk to Regis again.

Too late, karl is already dead.

In that case, you need to trek out to the Fort and nab the legion slave ledger from a desk in Caesar's tent, show that to Regis and he'll be convinced of the Legion's nefariousness.