I never heard of this either. I double-checked the NOAA weather radio website and there is no mention of an impending switch to all-digital.

There already is a digital signal embedded in the normal VHF analog signal as part of any Emergency Alert System activation--that burst of static at the start and end of alerts. Among other bits of info, that's what allows my weather radio to display a brief description about what kind of alert is coming through and when it expires.