Fellow blogger Gary McKeown on Northern Notes gives a great synopsis of the NIO decison to scrap the substitute list system for Assembly Members. The NIO announcement essentially means that if an MLA leaves office he will be replaced by the party's nominating officer rather than the person who is stated on the MLA's substitute list.
All I would add to Gary's post is that the last time the substitute list was used was in May 2008 when DUP MLA George Dawson died and was replaced by Alastair Ross. The Speaker Willie Hay had to proceed done the substitute list compiled by Dawson and ask if the person would accept the role. Ross, top of the list, accepted and thus is the only current member of the NI Assembly who was not elected by the public.