I'm not sure it's a simple as that. In the case of 3+ modified to 2+
If you had to reroll your 1s and 2s it's not 35 out of 36 anymore. I'm not a maths Guru but I think the fact that in the first round of rolls if you were forced to re-roll your 2s the odds would be slightly worse.
I misunderstood. This situation you described is not "2+ Rerollable" it is something else. Let me describe how the rules work in situations like this:
Scenario A: Model has Ballistic Skill 2+, and is going to move and fire a Heavy Weapon, giving it -1 to hit. It has an ability or is affected an ability that says that says "you may reroll all misses"
In Scenario A, you roll a d6 to hit and you get a...
1: You MISS, so you can re-roll.
2: Unmodified, you HIT, so you don't re-roll, and after modifiers you MISS.
3. You HIT.
4. You HIT.
5. You HIT.
6. You HIT.
Effectively, this is "3+ to hit, rerolling 1s" which will hit about 7/9 of the time.
Scenario B: Model has Ballistic Skill 4+, and gets +1 to hit from a special rule. It has an ability that says "you can reroll all misses"
In Scenario B, you roll a d6 to hit and you get a...
1: You MISS, so you can re-roll.
2: You MISS, so you can re-roll.
3. Unmodified, you MISS, so you CAN re-roll if you want. However, you choose not to, because you know that after the modifier this is a HIT.
4. You HIT.
5. You HIT.
6. You HIT.
In other words, with a positive modifier, sometimes you choose not to take rules that you rules-wise have the option to, specifically when the unmodified roll is a miss, but you know that with modifiers it would be a hit.
It's all very simple, in my opinion: look at the raw unmodified die roll to determine whether or not you can re-roll, and use the modified die roll at the end to determine whether or not it is a hit.