It may not really solve your problems - FCC Narrowbanding
True on the one license, HOWEVER, each license states the number of transmitters, if you are running less than your licensed number, you should be good.
I don't know about grandfathering, unless your license is already compliant, I don't think so.
UNLESS you have a 12.5 kHz radio, the only way to meet the new rules would be at 19,200 baud with an older 25 kHz spacing radio. It is yet as unproven in my mind if that is going to be a viable RTK rate, as far as the FCC is concerned you meet the law, BUT if range is reduced or there are other problems, then it does little good, I am researching this myself, I have not tried it to see if it even works at all.
Be interested to hear what the FCC has to say if you can find somebody there to talk with directly.
SHG
FCC Narrowbanding Mandate - update
I reached the FCC by phone this morning, and got the good news that our license is already compliant for the narrowbanding mandate (our first Emission Designator starts with 11K2).
Now, I need to learn about 2 more parts of this complicated mandate:
1) how to set up the TCS2 and R8 GNSS Model 2 receivers to use the 19,200 baud rate
2) how to check the radios to see which Emission Designator they are set to.