As comparing the stats of the NPCs of the old BRP-based A!C to the 2d20-based one, it would require ENORMOUS work to convert them to 2d20 stats, and you still need the rules for NPCs, their special abilities, magic, etc. given in the A!C 2d20 GM book to do that.
The A!C players book only has the player facing part of this system, which is not nearly enough to run it as a GM.
2d20 is a very different type of rules system from BRP/CoC or Savage Worlds. Knowing the old edition does not help at all in any rules and stats related regard.
You can easily use the old editions for the fluff, the setting information (which I actually recommend, as the A!C 2d20 is very thin on the setting background information). But slight differences in fluff does not bring the game to a grinding halt, rules mechanical difference do.
And, the old BRP A!C has been less “pulpy” than the Savage Worlds one, and A!C 2d20 is even more pulpy. That is, why the players in the different groups I ran and run A!C for prefer the new 2d20 version above all others.
It is VERY different, the gameplay is distinctively different using the 2d20 rules - especially from the GM’s side, having to manage the Threat resource, which is for many GMs a new thing and requires a bit of advice and more of actual play practice to get it going.
My recommendation: get the A!C 2d20 GM book, give it a very thorough read, especially if you already know the older editions, as the differences are important.