Mr. Pike. The FL models you have are perfectly fine and will be serviceable for eternity thru Vexilar. The FLX series has the same features yours has except the new brushless motor makes the display more crisp. Should you trade, I see no need to.
As far as the FLX28, The FL20 and FLX20, with the addition of the DD100 for digital depth, will give you virtually the same machine as the FLX28.
If I were in the market for a new flasher I would get the FLX20 in Pro or Ultra pack, which included the DD100 for digital depth.