I have converted my d20 stargate game over to use damage save instead of hit points. Here are the things i did...
Classes: instead of having hit die, these need to get damage save progressions. give thee d12 guys the fastest, then d10 etc. Now, stargate was simple since all classes had d8, d10 or d12. The numbers i am currently using are d8 = +1 damage save every 5 levels(so the range is from +0 to +4), d10 = +1 damage save every 3 levels (range from +0 to +6), d12 +2 damage save every 5 levels (range from +1 to +8.) This was intended to create noticeable but not overly invulnerable saves.
Weapons and Damage: Damage does not swing anywhere near as much in stargate as in DND. A high level guy's gunshot will be about the same as a lower olevel guy. Give every weapon a single damage rating and expect the damage save to be your randomness. DND will have much greater swing as the +5 sword with the guy with 26 strength is a lot more than he swung at fiurst level.
Feats and Abilities: i let sneak attack be +2 to damage per die of sneak attack. I let toughness turn into a free "damage save reroll" per session (to avoid multiple toughness making you bounce damage)
The SAVE chart: to add more flexibility my chart has more categories:
fail by 1-4 = hurt so the pain causes -2 to all actions for 1 round
fail by 5-8 = shook up pian for -2 AND lose next half-action
fail by 9-12 = stunned for 1 round
fail by 13-16 = scene kill: for the scene withpersisting probeblems afterward
fail by 17+ = mission kill: out for the adventure and probably dying. This should "force" or encourage retreat to get help.
Death is deliberately left vague... anything in the mission kill can be lethal depending of the circumstances. PCs moving to help immediately swings the balance.
HEALING: Stargate has little healing of extraordinary means so it was easy. For DND, I would have the spells start only treating the lesser effects and growing in power to cover the more serious. maybe a CLW is not able to help wioth a scene kill no matter how many you throw and you need a cure serious to bring someone out of that quickly.
ARMOR: In MNM everything is a touch attack and armor reduces the damage save dc. In DND thats AC. So that will be your biggest change. Its a good one, but it means your PCs will hit a LOT! What you probably want to do is add in as well a Defense Bonus, or BDB, that progresses like BAB does. The typical ones i see have the highest BDB looking like the cleric BAB, 3/4, and these go to your rogues and such, most running ay 1/2 like the wizard BAB and this covering your typical mid road fighter and such, and the lowest being at about 1/3 and that going to your non-coms like wizards and sages. You could even not give commoners a BDB and help show them as the fodder they deserve.
The huge thing you will have to think on and figure out is where you want the balance between attack and offense to be. For stargate for instance, i want most shots to be very dangerous and it to be RARE if ever that a shot bounces. So my typical gunshot DC is like 28-30 and the typical armor bonuses being +3-5. of my four PCs, getting hit with one round from a P90 SMG will mean they get a +7ish d20roll vs a DC of 27. 1 in 20 chance of bouncing the shot off armor, 4 in 20 hurt, 4 in 20 stunned, 4 in 20 scene kill, and 3 in 20 mission kill. They dont want to get hit. (Now, lucky breaks/hero points/action dice allow for rerolls as does toughness so one bad roll wont be killer, but its still no walk in the park.)
Its up to you to figure out how lethal you want your DND to be. Epic fantasy is not modern scifi and a dwarf taking a axe shot and still going might be very apropos so you might want more MNM levels where your character bounces most attacks.
What you need to do is generate a fifth level guy, a 10th level guy and a 15th level guys and some reasonable threats. Then look at their saves and attacks and see where you need to set damage values to get your preferred feel.
Thats all for now, off the top of my head.
BTW, Green Ronin is releasing BLUE ROSE, a fantasy game d20 later this year which will use the damage save mechanic.
hope this helps some.
