It depends on how you are controlling the 1336 presently. I worked in a plant where we upgraded to PowerFlex 700 pretty successfully, but it is definitely not a drop in replacement.
Give us more info:
1. How many drives?
2. What HP?
3. Are you using RIO, ControlNet, Devicenet or another comms interface with the old drives?
4. Also, there are a few variants of the 1336 out there, so the exact type of VFD you already have might be important too.