According to my knowledge, I don't think that is possible. But it might be in the future. If you want to beat them again for fun, you can play offline singleplayer, because the game only recognizes your achievements when you are logged in.
Start on a new computer. The Stats are the same but the Achievements are not. If you don't want to do that, or can't, do as they say, though the stats will get deleted with it.
1. Create Folder in .minecraft: Stats and Achievement Saves
2. Create more folders in that, 1, 2, 3, 4, 5, 6 ,7
3. Drag stats.dat into folder 1
4. You now have made a stats.dat save file.
5. Go onto minecraft.
6. You now have another stats.dat!
7. drag that into folder 2.
8. You have saved another stats.dat.
9. Drag an stats.dat out of it's folder and into .minecraft to load.
10. TIP: set your jumping key to Caps Lock, keep Caps Lock on and don't move to
keep earning more on "Jumps"!
2. Create more folders in that, 1, 2, 3, 4, 5, 6 ,7
3. Drag stats.dat into folder 1
4. You now have made a stats.dat save file.
5. Go onto minecraft.
6. You now have another stats.dat!
7. drag that into folder 2.
8. You have saved another stats.dat.
9. Drag an stats.dat out of it's folder and into .minecraft to load.
10. TIP: set your jumping key to Caps Lock, keep Caps Lock on and don't move to
keep earning more on "Jumps"!