I have a $1900 credit with CO for an EWR-ZRH trip in BF (Z class) that I canceled last month. When I called to use the credit for EWR-LGW today, I was told that if I change the destination, I would have to be charged the same fare type or higher, plus $300 fee.

I knew about the $300 fee when I called to cancel, but wasn't informed that if there's a new destination, the same fare bucket or higher rule applies.

So now if I want to change to EWR-LGW, ticket is $2500 plus $300 fee. If I keep the EWR-ZRH route, it is $2700 plus $300 fee.

Any good suggestions how I could best use this $1900 credit without spending more?
:-(

I guess I am so used to DL change policy, where if you have a $1000 credit, they deduct the $200 change fee and the $800 can be used for any trips with Delta.

Call back. You can use it for a less expensive trip, and ask them to issue an ETC for the change. Note the ETC is one-time use, so keep that in mind.

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.

It took THREE different agents, 92 minutes on hold (YES, NINETY-TWO MINUTES) and FINALLY I was able to get the lower fare on different routing with an ETC issued for the difference.

Thank God for SPEAKERPHONE and thank you Channa for suggesting that I call back.