Around here it seems that 27 band is filling up. There have been times when people get bumped out of a main because there's seven guys on 27 and only 6 channels. My question is can 27 mhz DCX receivers be re-tuned to 75 mhz?

You will need to buy a new Rx on 75MHz.....

Does your radio use modules?
If so, you buy just the Tx module to change to 75MHz from 27MHz....

hope this helps....