To convert a den to a room, you will add a closet, which will most likely require an addition and/or a relocation of light switches, outlets, and potentially ductwork for the HVAC. 

When the new HVAC system and duct work was installed, some of the joist bays were used as return ducts.

No duct work currently.  

I am considering replacing.Here is why:  Years ago I bought a home and replaced the furnace and all duct work etc.  

-I don't know about how the duct work is currently set up for ac/heating of this room. 

. - doesn't require new ductwork.

Renovations included the following:•Previously one electrical meter and panel box, installed 4 new meters and 4 new panel boxes, complete rewire of entire building•New outlets, switches, and light fixtures•Repaired foundation•Split gas into 3 meters•Brand new ductwork, furnaces, and Central AC for Units A & B•New gutters & downspouts•Roof, flashing repaired•Trees around house removed•Entire exterior painted•30 windows replaced•2 new water heaters•Replaced about 75% of plumbing•Cleaned out garages, installed divider, and installed new garage doors•All 3 units:• New floor & paint, drywall repair• New kitchen cabinets, counters, appliances• New toilet, shower/tub, vanityPurchase price: $85,000Renovations: $175,000All-in, including closing & holding costs: $270kAppraised at $306,00080% LTV cash out refi (cash back after fees): $236kSo I'm left with about $34k in the deal. 

I'm spending about $5K to replace the AC, furnace and duct work.  

Of course, an experienced contractor would bring this issue to you in advance and resolve it but he overlooked the fact where a ductwork for a central air is not feasible.

If not, have insulation contractor spray several inches of cellulose. 2) be sure the duct work is not leaking air.