They are unlocked.

Until the current models, there were separate SKUs for different carriers, with different modem configurations. This year, all the LTE bands are the same and there is just a single LTE SKU for each model.

So when you get an AT&T or T-Mobile, all you're getting is the SIM included from a carrier.

That will let you subscribe on a month to month basis right away, since you already have the nano SIM. Then you buy a month worth of access and decide if you want to keep buying.