Can I exchange a chimney flue for a larger diameter flue?
first thing to understand is gas appliances use different materials and protocols than wood burning appliances.  they are not interchangeable.

if you have a preexisting ceramic flue or stone flue thats large enough for the new liner, then it should be pretty simple.  if its going into a ceramic or stone flue, it doesnt need to be double walled in the chimney, just if it passes out into the room (as a lateral or vertical connector).  its pretty common here in ontario to use 8in or 10in flexible stainless flue liner pipe to do these type of retrofits. however, it could be smaller for a smaller unit.  some zero clearance units have exhausts of 6in.

if you have to install a new larger flue, it will be very costly and invasive as it will essentially mean replacing the entire chimney column from the basement flue port up to the roof.

i would start with calling a licensed fireplace installer to come have a look.