Cancel order

So I got a preorder through ATT and the ship date is 14-21 days. I just found out that Walmart will have preorders for pickup on launch date. I want to cancel my ATT order, but do you think that somehow that will screw up my subsidized purchase somehow?

And by "screw up", I mean any lag before walmart knows I cancelled so they don’t try to say that I already got my discount and not recognize my cancellation.
I mean... it IS Walmart and ATT. This combination is lethal.

I just don’t want to be told I can’t and then by the time it’s corrected, I miss out on the Sep 21st pickup and then possibly miss the 14-21 days and potentially delayed like 3 months now.

Ugh, 1st world issues.