Single bedroom ventilation - Removing external vent/grill

[mike2016]

I'm considering installing a single bedroom ventilation unit, something like the Blauberg Mini-Air or similar as my bedroom gets very stuffy at night. I live beside a main road. The existing passive vent is mostly blocked except for a diy z vent channel designed to keep out noise but very little air makes it through without a tailwind. 

 

My question is around installing the external part of the Blauberg, I need to remove the existing grill. It's at 1st floor height and I've only a ladder. I'm weighing up the risk of chipping away at the render at this height and figuring out how the grill is held in place so I can remove it. Not something I've ever done before. Or I multi tool out a 104mm circular hole (seems to be metal) for the vent pipe and fit the new cowl over the old vent and fix in place? I can access it from inside if so to do that much easier. 

 

The old grill is 200mm (w) x 150mm (h), the new cowl is 175mm (w) x 222mm (h)

 

Any ideas? Thanks.