You are right in your interpretation of the info from Futaba. You are not supposed to change the crystal in a transmitter module without having it checked for frequency accuracy and output power.

You can send the module to Hobby Services and they will change the frequency for $25 plus $9.99 shipping
http://www.hobbyservices.com/futaba-rates.html
There are independent service centers who can do the same job. Radio South is well known.

If you choose to buy a new module:

This is the fixed frequency module. Price is about $50
http://www3.towerhobbies.com/cgi-bin...I=LXH256**&P=0
If Tower does not have your channel in stock, check http://www.servocity.com

There is also a synthesized module that does not use crystals. You turn little dials to put it onany channel from 11 to 60. Price is about $100
http://www3.towerhobbies.com/cgi-bin...?&I=LXHKB1&P=0

<<EDIT >> If your 6EX still works, having the existing 9C module changed to your channel makes the most sense to me.