I called Best Western at 1-800-444-7646 and spoke to a rep who took all of my info on the phone. You must have an existing Best Western Rewards Account to qualify. I did not need to provide proof of status. I was given a status match from Hilton Gold to Best Western Diamond.
FYI: The match is through this calendar year. To keep the Best Western Diamond Status in 2019, all I need to do is call and provide my status at Hilton (or any other program) and they will match it again, if I don't meet the Diamond Status requirements through their program.