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Can I narrow this air return opening?

last month

We're in the middle of a major home reno, which has included adding a second storey (560 sq ft) and a new higher-efficiency gas furnace. Lots of additional ductwork for the second storey running down to the furnace room. This air return (in the pic) on the ground floor, used to be the only air return vent in the house. Now we have 3 additional air returns on the 2nd floor as well as this original one on the main floor. (The green arrow represents where I want to gain an additional 2".)

This air return hole is 30.5" wide by 10". Two questions:

  1. Can I reduce the opening slightly e.g. from 30.5" to 28", without negatively affecting my air circulation?
  2. Is there any chance that reducing this air return opening slightly e.g. from 30.5" to 28", without getting sign-off by an HVAC engineer, could be a Code violation? (I'm in Ontario, Canada, BTW).

Thank you!

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