The last time I changed a flight on CO, they charged me for the change fee and then let me use the full amount of the credit toward a new ticket, as opposed to other airlines that deduct the change fee from the credit and let you use the balance. However, I think it will depend on the rules of your original fare.
There are fares that specify "IF THE NEW ITINERARY MEETS THE PROVISIONS OF NOTE 3 ABOVE BUT THE FARE RECALCULATION RESULTS IN A LOWER FARE THE DIFFERENCE IN FARES MAY BE APPLIED TOWARDS PAYMENT OF THE USD 200 CHANGE FEE. ANY ADDITIONAL REFUNDABLE AMOUNT WILL BE REFUNDED IN THE FORM OF A NON-REFUNDABLE MCO OR TRAVEL CERTIFICATE VALID FOR FUTURE TRAVEL VIA CO SERVICES. "

Some fares specify: "WHEN THE ITINERARY RESULTS IN A LOWER FARE THE USD400 CHANGEFEE WILL APPLYAND NO REFUND WILL BE MADE. " which I think implies they will charge you the change fee no matter what.

In terms of that as well as the "same fare type or higher" issue (which I've never heard of), I think you need to get a copy of your original fare rules. Always a good idea to make a screen capture when you book online, otherwise you'll have to rely on the customer service agent to read them to you over the phone.