Impressive work guys!  Watching Mike's 2012 talk on youtube was what really got me excited about bitcoin again and I'm glad to see such amazing progress in making these concepts a reality.

I'd be delighted if someone could elaborate on how things change once transaction replacement is turned back on.  From what I've gleaned there's a risk of the refund transaction being modified and broadcasted but I don't understand how this could happen before the locktime and without the server resigning the inputs.  What exactly is the caveat with this implementation and transaction replacement?