I want to replace my 2 1/2 ton ac unit with a 4 ton. Will I have to replace the duct work, etc. in order for this to work?
Yes, you should scrap old ductwork and install new correctly sized and insulated properly.
You will also require a matching new furnace correctly sized with a 4 ton rated blower along with matching evap coil for the new 4 ton condenser.
Correct sizing and a completely matched system is mighty important.