You may want to keep costs down by keeping the old flex, however you may be putting your families health at risk. A new fan blowing more air, movement of flex in a roof during installation can dislodge collected and formed dust with the existing system blowing this into the conditioned space of your home putting your families health at risk and providing a poor indoor air quality environment.
The existing flex of a previous installation will be old, possibly brittle and likely to have a low insulation rating. When installation new ac system - the newer fan will provide a higher and more efficient static pressure within the duct.
The existing flexible duct and the entire installation would be close to the end of its usable life. As higher pressured air moves though older flexible duct it can easily pressurize so much as to break existing duct taped seals, find a weak spot and create a blow out where air seeps into the ceiling space or energy efficiency lost due to lower insulation ratings. For residential applications the minimum insulation thickens needs to comply with state requirements.
The thicker the insulation the less efficiency is lost through the penetrating hot/cold ceiling space into the flexible duct.