You say that you believe it to be one of the originals; a quick check of the date code on the gun will help narrow that down.
The recoil assembly was swapped for the heavier .40 version as mentioned above, whether or not it's an improvement depends on who you ask. The other change was the trigger bar spring assembly due to some adhesive failures with the early models but by all accounts that's only an issue if it is damaged during improper disassembly/reassembly of that specific part, which is something that should only be done by an armorer anyways.
Despite what some people say, the VP9 is a phenomenal pistol and is on-par with the other modern HK pistols.