you don't NEED different fins to go freediving, anything will work. Some will work better. If you want to get new fins for freediving, you should first think about how into it you want to get. You can use the fins you have and be ok, your scuba fins will do the trick for some light recreation, but if you want to get even moderately serious look into some entry level (ie. cheaper) free dive specific fins such as
Omers or
Cressi Long Blade FreeDiving Fins. Then when you are ready look into the high end to really get something out of your fins
C4 Mustang ($$$).
Just be very aware, these are very large fins, they are hard to pack and travel with, and some can easily be broken
My personal recommendation for a scuba/freediver is to get fins that work for both scuba and freediving, they may not be quite as good for freediving, but you'll be able to use them in both arenas very well, and only have to pack 1 set of fins. I personally used the
ForceFin Extra Force for years and love them, although now I like the
ForceFin Excellerators more, in fact most of the ForceFin line is pretty good, and the Apollo Bio-Fin isn't terrible.