destroying terrains

[exid]

PvsS rules says that when you cast a terrain over another terrain, you destroy the old one paying it's cost (+ conjurations attached to it)... and nothing about battleground's terrains.
The supplement says that you destroy the old one except if it's a battleground's terrain, that cannot be destroyed... nothing about paying anything.
And I read somewhere on the forum that battleground's would be destroyed with no cost...

What is the final rule?

[iNano78]

PvsS rules says that when you cast a terrain over another terrain, you destroy the old one paying it's cost (+ conjurations attached to it)... and nothing about battleground's terrains.
The supplement says that you destroy the old one except if it's a battleground's terrain, that cannot be destroyed... nothing about paying anything.
And I read somewhere on the forum that battleground's would be destroyed with no cost...

What is the final rule?

It's explicitly described in detail in the Battlegrounds Domination rule book. I know because I was rereading it this morning...
http://www.arcanewonders.com/resources/Mage_Wars_BGD_Rules.pdf
(see p.3 in a brown box)

Terrain

For future compatibility, each of these special tiles counts as terrain. Terrain
is a type of conjuration spell players will be able to cast in future Mage
Wars® expansions. A terrain spell changes a zone, giving it special new
features. For example, a “Mud” terrain spell might turn a zone into a sticky
pool of mud, whereas a “Hellscape” terrain spell might create a hellish fire
zone.

When you cast a terrain spell on a zone which already has terrain in it, the
new terrain spell cancels out and replaces the old terrain spell effects. The
special tiles of Battlegrounds all count as terrain, and you can cancel their
effects by casting a Terrain spell on top of them. However, they cannot be
destroyed or moved.

You must follow all of the normal rules for overlaying Terrain. For the
purposes of using terrain rules, these special tiles have a mana cost of 0.
If, for some reason, the Terrain that overlays a special tile is removed or
destroyed, the original special ability of the tile functions again.

The mana cost of a Domination terrain is "0" for the purposes of covering it up (e.g. it only costs 5 mana to put a Shallow Sea overtop of any one of the Domination terrains). And the new Conjuration - Terrain replaces the effects of the Domination terrain. However, it is never "destroyed" - e.g. if somehow you could remove a Conjuration - Terrain, the Domination terrain would return. The only way this can happen currently is if a Mage is killed in a multiplayer game, since objects controlled by that Mage get removed from play.

[Zuberi]

iNano78 has it exactly.

[exid]

  i didn't thought to look in the domination rules... sory!