It's true that CBM is very pure (better than 98%, IIRC). As you state, this means it has a lower calorific value than regular natural gas, not to be confused with Lower Calorific Value (grin). This doesn't mean it's not marketable. My domestic gas bill is quoted in megajoules, not cubic feet, meaning the gas supplier corrects automatically for the calorific value and the customer pays for energy received. So that particular problem has been solved.

Another thing to take into account is the Wobbe Number, which is a function of upper calorific value and density http://www.sizes.com/units/wobbe_number.htm - as described it is not a dimensionless number. According to the website, methane falls right in the middle of the Wobbe Number range for commercial natural gases in the United States. IIRC the gas from the Morecambe Bay field (offshore UK) had an out of spec Wobbe Number and had to be blended with nitrogen before it could be put into the National Grid.

If you change to a gas of dramatically different specification then you may have to change the burner tips to avoid flame blow-out. This happened in the UK in the late 1960s with the switchover from coal gas to natural gas ("High Speed Gas"), and was a huge logistical effort, though the world doesn't appear to have come to an end as a result. If the gas goes to a single large user (like CO2-rich Miller Field gas in the UK) then it's simpler.