This is the RX in question
http://www2.towerhobbies.com/cgi-bin...&I=LXGFR5&P=FR
As you can read, it IS hi/low sensitive and to cross over without a retuning will cost some RX sensitivity (range) issues though they may/may not be great.

Ken has quoted the FCC rules properly and this is also the advice given by BAX who is the American spokesperson for Futaba.