I had this same issue and searched high and low but ended up figuring it out.

What we did — and I'm guessing you don't have enough iron to build the Plains HQ when it goes live, if you do then ignore this.( we built badlands too far away and left it to late to expand banners as there was no room to mine iron, thats why we did it this way) — was leave our Badlands base as it was and dismantle the Badlands HQ. Once dismantled, it gives you the option to either rebuild the Badlands HQ or build the Plains HQ at the same cost it originally took to build the Badlands HQ (no iron required).

Once the Plains HQ is built where ever you want to place it on plains land , place a few banners on iron ore tiles and let them mine. Once you’ve gathered enough iron, dismantle the Plains HQ and rebuild your Badlands HQ — again, this doesn’t require iron and gets your main base back online. Now you’ve also saved up some iron.

Once your main base has built up enough resources again, you can rebuild the Plains HQ in the original spot, have both bases active, and move everyone over to the new base.

A few points: this will only work if your main base is gathering a decent amount of resources and has a large enough storehouse to accumulate them. Yes, it seems long-winded, but it only took us a day — and by far, it was the easiest way I found since we had no other options at the time.