I was thinking such mod will be cake to do, but it is much more tricky than I expected.

What I want to do:

1) In battle, player clicks custom button, dialog shows and offer possibility of changing battlefield terrain and top terrain. Effect is instant, both visual and creatures bonuses.

What it does right now:

1) Effect is not instant, player has to move a creature before changes are visible.

2) It did not work if battlefield is regular (not changed by user in BA trigger first), but Gamecreator provided me a code which does the job very well, thanks

3) For now you can click once, activate one battlefield, close dialog. But if you select "cursed ground", close dialog, open dialog then choose "grass" you see grass but penalties are still those of cursed ground.
This is because top terrain remains even if the base terrain is changed. Should I use other mechanic? Perhaps restrict mod to only top terrains? Or allow double selection, one for top terrain, one for base terrain?

4) I can't solve the native terrain bonuses. Even if I change the terrain to not native, the bonus stacks had at battle start (+1 attack, def and speed) will remain. Is there a way to get rid of? Because coding this manually would be a hell. (tried with BA:B# but it has no effect)

5) I don't seem to be able to trigger the DL it from new button (you see it bottom left), I get 0-400 as y3/v2 so for now it triggers from right-click on system options. Now I understand why nobody yet added a button to battle: it big sucks.

Battle-Mod (4 mb)
If any can help, please.

Code triggering the BF change: