Trying to make a life-long Medigap choice because I'm started on SSDI before age 65!!

Spoke with an agent today with Boomer Benefits and he told me that because I already have Medicare because I'm on SSDI (and turning 65) that I will never be able to pass underwriting to switch medigap plans once my choice is made when I turn 65. He told me that one of the underwriting questions they ask is whether you started receiving Medicare before the age of 65. This makes my choice of G or N or HDG a lifelong choice.