It's plausible, but it's not the straight forward effect that you might expect. Consider some opponents, whose AC you could hit exactly with a natural 1, 6, 11 or 20 without any concealment.
By the RAW, when they have total concealment, you'll hit the first one 47.5% of the time (0.5 * .95), the second 37.5% (0.5 * 0.75), the third 25% (0.5 * 0.5) and the last one 2.5% (0.5 * 0.05). With the new rule, that would be 50% (hit on 11), 25% (hit on 16), 5% (hit on 21 on natural 20) and 5% (hit 30 on natural 20). (Is there a decent way to create a table here?)
In the usual range where you need somewhere between 5-15 to hit, it makes concealment much more effective. There's a similar but less drastic effect for the 20%/+4. If you have very low AC (wizard who's just been debuffed), it makes concealment much less useful.
You'd also need to change blindfight, since there's no longer a miss chance to reroll. I'd probably give a +4 bonus to hit against concealment and about +6 against total concealment.
