Change of Heart
If you have an Uncommon item or higher you can upgrade it by Hitting Y when looking at the item in your gear. If you use 1 item long enough then you unlock attachments for it.
If it has more than 1 scope attachment, simply change from scope to scope and the achievement will unlock.
You can also change your grenade load-out and it will unlock.
Its buggy as I have done this and it hasn't unlocked.
Edit: It just unlocked when I started the game up so it might not unlock right away, but if you do what I said it will.
Video is done by Alex LegendForever, this is when he changed his Grenade ability that unlocked the achievement.