It looks as though the new pebble steel has a new type of watch band, unlike the original pebble which had a standard size (22mm) watch band that you can replace.
This means you will only be able to buy a replacement band either directly from Pebble or from 3rd part accessory makers specifically for the pebble steel.
So far we haven't heard any details from Pebble, but this is certainly one of the biggest downsides of the pebble steel.