The thing with pattern matching is that it is as safe as the OO approach, in the sense that the compiler will warn you if you don't match all the cases and that forcing you to use an explicit ADT protects against boolean blindness (for example, writing code that assumes you are in case 2 if you are not in case 1)
That said, when it comes to extensibility, pattern matching is more similar to if statements then the OO - its easy to write new functions but hard to extend the original ADT with new cases. If being able to add new functions is important it might be better to use if statements then to do a major rewrite to use OO instead. (You could always use a visitor pattern if you want the extra compiler safety but that can get very complicated, IMO)
That said, when it comes to extensibility, pattern matching is more similar to if statements then the OO - its easy to write new functions but hard to extend the original ADT with new cases. If being able to add new functions is important it might be better to use if statements then to do a major rewrite to use OO instead. (You could always use a visitor pattern if you want the extra compiler safety but that can get very complicated, IMO)