Is it possible to change the TileId of a pinned secondary tile in Windows Phone 8.1 RT? I tried it via tile.UpdateAsync() method. But I was only able to update arguments. Anyone?
No, you cannot change the TileId. This is called out in the UpdateAsync docs.
Since creating a tile must be approved by the user the unique identifiers (the ID, logo, and the DisplayName created by the user) aren't modifiable by the app. Changing those identifiers is essentially creating a new tile, so you could request a new tile be created.
If you want to remap the meaning while leaving the same(ish) tile visible you can update the arguments and general appearance.