This achievement can be unlocked at
level 7 as you'll be granted the ability to change your grenade at this point.
To do so: press
then
on your sub-class (the big orange/purple/blue square in the gear menu) then simply select the 2nd grenade on the left. This will change your grenade thus reversing a decision you made in the upgrade grid.
On this picture, the player has selected another grenade than the first you are allowed to unlock.