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I ran a regression on 5e AC's and AD&D AC's, and came up with a conversion formula:
5e_AC = -1.25*(AD&D_AC) + 15
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Edit: since this was done the Real numbers, you can round appropriately, or not.
That formula clearly doesn't work. 10 in one system should still be 10 in the other -- that is the AC of an unprotected human with average dexterity in both systems. Your system would convert an AD&D AC of 10 to 2.5 in 5E.
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You are making it too complicated. Try this:
AC(5e) = 20 - AC(AD&D)
AB(5E) = 20 - THACO
According to the playtest packet from last year:
Padded- 11
Leather- 12
Studded leather - 13
Scale mail - 14
Chain mail - 16
Plate Mail - 18
I really don't feel like digging my First Edition PHB out of the closet, so I'm going by memory here. Feel free to correct me if I'm wrong:
Padded- 9
Leather- 8
Studded leather - 7
Scale mail - 6
Chain mail - 5
Plate Mail - 3
So there is not an exact translation there. If you were to use jw70's formula then Chain and Plate Mail would be 15 and 17 AC in the newer system, but the others work pretty well.
As for monster AC. I'm pretty sure that the play packet would be all over the place compared to first edition. The monsters in the last packet were far from finished, but most of the design will be based on math (I assume how much damage per round a standard party can likely inflict will probably determine the AC and HP of a creature based on what level it is meant to be). So I doubt it is possible for any formula to work for all creatures. A lot of the old stuff was really just "make it up as you go along". That just isn't how games are made these days.
That formula clearly doesn't work. 10 in one system should still be 10 in the other -- that is the AC of an unprotected human with average dexterity in both systems. Your system would convert an AD&D AC of 10 to 2.5 in 5E.
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My group has been playing D&D 5e with 3 as the lowest AC since the 5e
playtest started. --> What page of the rules does it say 10 is the lowest?
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How do I convert the saving throws?
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My group has been playing D&D 5e with 3 as the lowest AC since the 5e playtest started. --> What page of the rules does it say 10 is the lowest?
10 isn't the lowest AC. 10 is just the base AC.
Without armor or a shield, your character’s AC equals 10 + his or her Dexterity modifier. If your character wears armor, carries a shield, or both, calculate your AC using the rules in chapter 5. Record your AC on your character sheet.
The lowest AC in 5e would seem to be 5. The base 10 for an unarmored creature, -5 with a Dexterity score of 1 (-5 modifier). There doesn't seem to be a size modifier, at least in the Basic PDF or the monster preview (no size modifier for the ogre or ochre jelly).
Likewise, the base AC in AD&D is 10, but a worse AC is possible.
How do I convert the saving throws?
You really don't. AD&D saving throws worked somewhat differently.
Basically, everything was against the equivalent of a DC 20. So if you take the AD&D saving throw and subtract it from 20, you can get a modifier. For example, a 1st level clerics saving throw vs spells was 15, so he needed to roll a 15 or higher. That is equivalent of having a +5 bonus on the check vs a DC of 20.
But that gives much larger numbers than 5e uses (such as 19th level AD&D clerics receiving +18 on saves against death or poison). So it is probably just best to ignore AD&D saves, and just give what is appropriate for the class level/monster in 5e.