I'm not that bothered about that part. Chances are that Microsoft will just convert them into cash and they will sit on your Live account waiting to be used much like your current points do. I doubt they will ever actually give it back to you.
The big question is how do you judge what a point is actually worth? Depending on how many you bought initially (800, 2100, 4200 etc) it could differ as the price per point comes down the more points you buy.
I guess the cost of content post change will need to be monitored to ensure that there is no sudden increases because they calculate the cost of content as higher then the cost of the points themselves.
I think it is a good move if they do it, there are just lots of issues around how they do it.