ALL frequencies will be considered narrowband.

The emissions on OLD wideband frequencies have to be narrow (11K) so they don't interfere with the new "narrowband" frequencies that are in between the wideband.

Its basically, the old Wideband frequencies (20k) have an emission that is wide enough to almost reach the emission span of the next upper/lower wideband freq. 

For example,
Wideband on 154.025, some transmitters may actually transmit 10K up and 10K down (a total of 20K) from 154.015 to 154.035.
Narrowband on 154.025, should be about 5K up and 5K down (about 11K), so transmits from about 154.020 to 154.030

The "new" narrowband frequencies between 154.025 and 154.04 is 154.0325 and the one between 154.025 and 154.01 is 154.0175.

If you look at the wideband freq "width", it interferes with the emission span of the narrowband frequency. (154.025 / 154.0325)
Reduce the old "wideband" to a narrowband emission and it will no longer interfere with the new narrowband frequencies.

People often think that "narrowband" is only the frequencies "in between" the wideband frequencies but not true.
Narrowband is just an emission type, and at this point, those are (almost) the one ones that are narrowband only.
(often frequencies like 154.0325  and 462.0375  - although some older UHF are still at 20K)

Thats why all licenses (and literally the radio equipment) (i think by 2013) have to be switched to narrowband (11K) even on the old "wideband" frequencies.

I believe agencies in the Chicago area have already experienced some major issues with their new "narrowband licenses".
Their output of input is just plain BLOWN AWAY from a wideband transmitter on the next frequency. 
Until everyone gets moved over, this could be  a major safety issue and personally would not suggest using narrowband yet.
**Just a personal opinion, I am more sure that with Motorola type radios, there is less of this type on interference.

See this for some help: http://wiki.radioreference.com/index.php/Refarming