I was looking at some ads for futaba rx and a couple of sellers were saying that you could change crystals and use either hi or low band on the futaba fpr127df. I thought that you could only switch crystals within the same band on this model. What is right?

As far as I know, the 127 was shipped with the high/low band tuning (I know all of the ones I have were). The risk you face by going outside the shipped band is that the reception will be reduced. So, it's best to stay within the band. If your receiver is a couple years old, it's not a bad idea to send it to a service center (Hobby Services, Radio South, etc) to be checked and tuned. Send it with the new crystal and they'll adjust it to optimum performance.

Futaba 127's have to stay in their original LOW or HIGH band. But you can make them any frequency within that band,

With the high cost of service to day, it is not cost effective to send a receiver of a plain four channel transmitter in for service. You don't by used ones at the swap meets either.

Throw them away and buy new if they give the slightest bit of trouble. Your plane/engine is worth more than a receiver.

The 127 is either high band or low band. Also, theoretically, you can change any frequency to any other in the same band. In reality, I like to stay within 2-3 frequencies either side of the original. If I go farther from the original, I send the RX off to be retuned.

Dr.1

I saw the same ad on a well known auction site. The seller is incorrect. While the R127DF is indeed a dual conversion receiver, you can't use low and high band crystals.
That would be true only of the R168DF receiver. Still, the R127DF was a fine receiver.

I didn't think the seller was correct. I have both rx and only the 168 stated it can be Hi or Low.

While having a reciever tuned is always a good idea, the three channel thing really isn't getting you anywhere. They are tuned from the factory for the center of the entire band (or the upper or lower half in the case of some futaba recievers) and not for the final channel.

I have changed crystals on the 127. The most drastic was replacing a ch12 with a ch 60. I very carefully range checked to about 100 yds. I had no problems, but if you try it, it's at your own risk. I didn't use these receivers in my most expensive aircraft.

A lot of folk's have done this sucessfully, maybe someone can chime in with a negative experience.