Ok so you can alter your stance by pressing "Left CTRL (Control)" + "AWSD" ... right there, that's awesome BUT...
I've had several recurring issues with this stance modifier so far...
Issue #1: Opening/Closing map will change the player's stance making him stand up then go back to a crouch stance (first crouch stance)
(This can prove to be an issue if you're on prone and/or trying to hide behind a rock using the lower crouch stance.)
Issue #2: Vaulting over an object while in crouch/lower crouch stance makes the player go prone then back in standing position to then vault the said object.
(Again... this can prove to be an issue)
Issue #3: Stance will chance if you're running and get exhausted
(If you run in crouch stance and get exhausted, the player will then get in standing position... this can prove to be an issue.)
I think that's it... there might be a few more issues which I've not yet encountered (or just forgot to add).