The digital signal will be modulated in band but right next to an analog channel. A radio station can broadcast both their analog and a copy of it digitally very close to it. As long as a station still broadcasts their analog channel, the old radios will work. When they stop the analog, the old radio will no longer see the channel. The FCC will probably mandate the analog stations for a while, but they will eventually go.
If you want to keep your old radio forever, someone will need to make a digital tuner that could be hidden in your dash with some controller (maybe a remote). The tuner could send the output along the coax to your old radio at a fixed channel like say, 104.5 MHz and you just leave your old radio at that channel.
Otherwise a retro looking radio like that website offers will be the answer.
The trouble with those retro radio places is they all seem to be 60s radios. Some time ago I found a Pyle radio that was 70s and 80s style with the dual shaft and the right look. I cannot find that one anymore and can't seem to find 70s and 80s style retro radios. Maybe more will come out some day.
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