Hardware solution for live video conversion
That or the UpDownCross converter should meet your needs as long as the frame rates are the same.  I'd personally stick with BlackMagic's hardware as you already have their ATEM and they make some of the most cost-effective hardware around.  They aren't the top quality brand for sure, but you won't beat the value for the price pretty much anywhere else.

If you need to change the frame rate, you will need some kind of frame buffer that can interpolate on the fly, which is likely going to get pricy.

Alternately, is there a reason that you can't vary the output of the laptop to fit whatever quality is needed for the feed from the A/V company?  This would avoid the need for a converter and seems to be the easier option.  A better output for the laptop may well be a cheaper solution.  The Intensity Shuttle for example is only $199 if you have a thunderbolt or USB 3 port.