I agree, had been in the same exact situation 2 years ago with App worked on by 20+ developers written in Clipper then FP 2.6. I think migrating to OOP is a good and exciting challenge yet would get you a lot of benefits in terms of speed, maintainability (code could be more compact), potential for enhancement and meeting new user requirements. The decisive factor is economics as you stated, whether you can convince the customer of the cost effectiveness or not.