Existing channels that are operating narrowband on the same frequencies will still be receivable, but the audio level will be lower as compared to channels that are properly demodulated. This constitutes the majority of narrowband channels at this time. You won't be able to tune precisely to the new frequencies freed up by narrowbanding (but you'd likely still receive them, albeit at a low volume level and possibly subject to some interference as long as you were tuned to the closest tunable frequency).