If you introduce this keep-your-champion-after-a-dodge you're just gonna see people dodging for other reasons, like when their teammates get bad champions.
If you happen to get a good champion but your teammate did not then it has the same consequences for your chance at winning, so you with the good champion can just dodge the game in the knowledge that you will keep your good champion and get another chance with a new team. This also makes keeping bad champions a mood point because its other people that dodge so the person with the bad champion will get a new one in the next game because he didn't dodge.
I also think it would be better not to increase the amount of rerolls you get, but instead simply always have 2 rerolls available every game instead of this stupid charge system. It's this charge system that can't deal with spurts of bad luck, I can go a lot of games never having to use any reroll but then a unlucky game comes along and I'm forced to use both, leaving me in the hands of fate for a good number of games. If we simply always had 2 rerolls every game this would not be a problem.
EDIT: I also think that giving 2 rerolls every match will lower the dodge rate, because everyone will be able to use them every game thus lowering the need to dodge games. At least somewhat.