Ok to reduce size of return vent grill?
Yes you should be concerned. When we size the duct work it is usually the minimum size needed. By reducing this opening you are affecting the furnace. Depending on the type you can cause enough excess heat to cause the safety’s to trip. Even if the safety’s do not trip the reduced flow can cause the fire box heat exchanger to run hotter (gas or electric) and fail sooner. So your plan needs to be modified a slight change in size 10% can cause problems but this huge change is asking for problems and I did not even mention it will cost more to heat the home because the flow is restricted.