"On each board" cares about the land the Dahan are currently in; it's fine to Push them onto a different board.
Like all instructions that don't involve targeting or "choose a land", if you
can obey them, you must - eg, if there's a land with 2 Dahan and a City that happens to be surrounded by Cities, you can't point at it and say "we would Push those, but we can't because there are no legal lands", just as you couldn't subvert "Add 1 Blight to a Sands" by pointing at a Wetland and saying "It's not a Sands, so we don't add Blight".
(Discussion elsewhere) Tags: Dahan, Events, Retreat (Dahan Event)