I searched around and found a
EnergySavers.gov page on furnaces and boilers.
One of the subpages is about retrofitting and older furnace, and I found this:
Quote:
Derating gas burners
Many boilers and furnaces in today's homes are oversized, particularly if you've upgraded the energy efficiency of your home. It is sometimes possible to reduce the heating capacity of your gas boiler or furnace to make it operate more efficiently by reducing the size of the gas burner orifice, and possibly also the baffles. This is a difficult process that should only be performed by a qualified technician, and in some cases, it could violate local building codes and void manufacturer's warranties. If allowed, though, the modifications should cost less than $100 and can save up to 15% of your fuel costs.
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This appears to answer my second question about reducing the size of my burner.