what happens with what use to be gems in games thought achievements

Those are just to show that the achievement is a rare achievement. It has nothing to do with the Rewards program.

What Darth said. Use a soul gem. Out of curiousity... what does Diablo 2 have to do with anything?? I'd also like to add, that even using your soul gems to recharge weapons increases your enchanting skill, which I thought was pretty cool.

Here are some specifics on how they could handle changing achievements, as Project Spark did. I'm almost certain they can change achievements after the fact, from a technical standpoint.

The trick is to make alternate unlocks that match the counter of the original achievement.
Project Spark did this to nerf many achievements. For example Hit Maker was 50,000 downloads. Now it's 50,000 downloads or "14 hours" of playing Community Games. But I believe a more accurate description would be "50,000 seconds" of Community
Games. 14 hours = 50,400 seconds. The devs could change the achievement counter to count both downloads and seconds played, and still have the achievement pop at 50,000 regardless.

I'm guessing Peak Performance is measured in meters or decimeters. So it should be possible to change the achievement to "Jump 100km or earn 100,000 / 1,000,000 Fit Points." Or perhaps just workout for 100,000 / 1,000,000 seconds.

I hope they do that. But from what I'm seeing they think they can't. Clearly they haven't seen that other games did it (Project Spark, Beach buggy racing & Killer Instinct).

Hey Xbox Fitness, why don't you check with them and see how it works?

I hope we will keet the achivements we earned so far, even after they kill the servers.