You always have the option of "bleeding off" excess air. This just kind of wastes some energy.
Another option during your "remediation phase" where you are wanting to get a lot of turnover is to simply tee in your old diffusers with the new. Instead of bleeding air off to the atmosphere, just split it up and run the excess through your old air stones and get some added water movement from them. If after trying it you feel it is moving too much water, take the stones out of the pond and use the bleed off method.
The other option is to simply use a different diffuser that can handle the volume, or add more of the Vertex diffusers. Maybe two doubles instead of two singles. Or if the membranes can handle additional air, drill out the orifice to a larger size.
Just some ideas to consider. Sometimes there is more than one way to skin a cat.
