DBFs tend to be the easy part, in my experience in a similar company (not as much revenue.) The spec is simple enough that one can write a reader/writer in an afternoon, even before AI.

My biggest challenge is the speed at which calculation heavy business logic can operate in FP, even on network shares, as long as they're optimized for IOPs not bandwidth. Trying to adapt similar data access patterns to a networked RDBMS will kill you with round trip and connection overhead. Converting that logic to set-based operations or stored procedures means altering logic and all the risks that entails.