Convert landscape proxy to landscape

I’ve build a level with world composition enabled (landscape). Now I need to convert the whole map back to one level with one landscape. I’m struggling to do so. Even If you disable world composition and move everything to master level - landscape proxies stay in place and if you try to delete or convert them - part of the landscape disappears.

I’m not keen on redoing mahoosive amount of work I did on the landscape so I really need to know how to convert those proxies back to one solid landscape or at the very least 8 landscapes.

Please help.