I have an object with sortable behavior and the use_scope attribute is set to true.
When i'm changing the scope_column value to an another on one item, the sortable_rank is broken, because the sortable_ranks aren't recalculated, so the move_up and move_down actions throw Fatal error: Call to a member function getSortableRank() on a non-object.
there is another issue try to fix this #359
I think you should call removeFromList() before changing the scope,
Thanks for the tip.