Well what happens for a retiming/rebook is that it doesn't immediately pick up on connection times before and after. So it may effectively end up giving you an impossible connection. What would normally happen is that at some point the system would pick this up and probably rebook you or send you an email to call. Of course if you spot there is an issue and know what you want to rebook to when you get the original change, you can just call to sort it out.