Adding Medicare Part B, general vs special enrollment periods

[NewlyDiagnosed]

I've been eligible for Medicare for several years but have not signed up for Part B because I had employer-sponsored health insurance.  For a couple of reasons I've decided to give up my current insurance and switch to Medicare.  Fortunately, this is the general enrollment period until March 31.  I've gone onto the website and tried to start the process, but it puts me in the special enrollment category.  I have not yet lost my current insurance (although I may soon because of funding issues) so I don't think I qualify for the special enrollment period.  Has anyone had any experience with this?