What is the current heat source for the high velocity system? Is it sufficient to use and keep the oil furnace off, or at least in reserve for the coldest of days?

I agree with Elmer. I suggest you explore options for making the high velocity system serve your needs and consider a separate system for the basement HVAC if needed.

It is unusual to see a house built in the 1980s with oil fired baseboard baseboard heat on the first floor and high velocity heating on the first and second floors. Have you tried heating the house in the winter with just the high velocity system?

Re: mike_home

“It is unusual to see a house built in the 1980s with oil fired baseboard baseboard heat…”

I would tend to agree but won’t comment further until the OP posts more info, particularly about the type of baseboard emitters (cast iron?) and water temperature required.

IMPO

SR