I find honesty is best way. Say that when you started, you made naive mistakes on the contract and in your business which you now know were poor business. That you value this client but the old contract is in conflict with everyone else and you would like to discuss the policy changes in the hope they will either agree to same contract as everyone else or you can negotiate to meet in the middle. It's okay to tell them that you don't want to have to let them go but you missed out so much keeping them, that it might be a consideration if they unwilling to change contracts.