To preserve the probabilities exactly, the new DC should be "14 + monster defense."
How I got that number
So, you want to convert this:
d20 + monster_save vs. 8 + caster_modifiers
d20 + caster_modifiers vs. ?? + monster_save
Here's how to figure out the "??" using a bit of intuition about probability:
- Ignore the modifiers for a second, since you'll be keeping those the same anyway. What's the probability of making a DC 8 check on a straight d20 roll? There are 7 values on a d20 (1 through 7) that fail, and 13 values (8 through 20) that succeed. So 13/20, or 65%. (Try
output d20 >= 8 in AnyDice.)
- Now, flip the percentages. You know you want the monster to succeed 65% of the time and fail 35% of the time (before mods). So that means the caster should succeed 35% of the time (7/20) and fail 65% of the time (13/20).
- In other words, you want the lowest 13 numbers on the d20 (conveniently, those are 1 through 13) to be failures. What DC is that? It's actually 14 (because a roll equal to the DC means you've beaten it).
- Thus, to preserve the probabilities exactly, the new formula is:
d20 + caster_modifiers vs. 14 + monster_save
- Try a few examples to check your work.
(This is a bit speculative because the game is unreleased, so we haven't necessarily seen all the rules, and some of them are likely in flux.)
Note that just being the one who gets to make a roll is sometimes a big benefit. For example, D&D Next playtest stuff had the concept of advantage, which lets you roll two dice and keep the highest. If defenders were previously able to claim advantage on saves, then shifting rolls to the attacker's side can make spells and monster abilities more powerful even though the raw math is the same.
Likewise, if the system features any kind of "bennies" or "action points" that you can use to enhance a roll, then shifting rolls from defenders to attackers generally makes combat faster and attacks harder to resist. Whether this favors PCs or enemies really depends on who's forcing more saving throws.