I'm one of those people that didn't have an issue with descending armor class. Maybe I should have, I dunno.
I also have nothing at all against the d20 mechanic and ascending armor class.
So what do you do in a retroclone? Do you follow the spirit of the old ruleset and have a To Hit table like OSRIC and Swords & Wizardry? You could use the table and manually write a "To Hit Armor Class 0" on the character sheet.
You could go the route of Dark Dungeons and use something like thac0 and a to hit table and come up with Attack Rank. Attack Rank A = thac0: 20, Attack Rank B = thac0: 19 and so on until you reach Attack Rank S = thac0: 2. It takes until Attack Rank X to be thac0: 1 and then it descends again throughout Attack Rank Z to Attack Rank XX. Again, you can pretty much convert an Attack Rank into a thac0, cross out the Attack Rank letter on the character sheet and substitute a thac0 number.
You could go the Basic Fantasy route and just use Base Attack Bonus and ascending armor class. In this case, you take your old material, perform a 10-AC conversion and make appropriate notes throughout that penalties to target's AC are really bonuses to attackers' to hit roll.
For now, I plan on using a Successful Attack Target Number or SATN. (considering the consternation created by thac0 discussions, I think the acronym is a perfect fit!)
The forumla will work like this:
Roll 1d20 +modifiers + target's AC >= SATN is a hit.
The beauty of this system is that when I see thac0 in my old modules, I treat it as the SATN. Thanks to the additive property of equality, the result is the same. Don't believe me? Look it up on any thac0 discussion. Also, let's follow this example:
Brad, a 2nd level Fighter has a thac0 of 19. He is attacking a goblin with an AC of 6. With a STR of 15, he receives a +1 to Hit.
Roll is 13. Add bonus to hit to increase the total to 14. Take thac0 (19) and subtract the armor class: 19-6 = 13. The roll of 14 is greater than 13, so the attack is successful.
now try this:
Brad a 2nd level Warrior has a SATN of 19. He is attacking a goblin with AC of 6. With a STR of 15, he receives a +1 to hit.
Roll is 13. 13 + 1 (STR Bonus) + 6 (Target AC) = 20. This is larger than the SATN of 19, so the attack is successful.
On the GM side of the screen, 2nd Edition monsters appears with a thac0. The GM does the same calculation as the players when rolling for the goblin.
The other thing this mechanic allows (you know essentially keeping the thac0), is that this allows fiddling with monsters. More on that later.