Roll Over - you can use this function at any time right after settlement (not before). Roll over - is just function to extend the validity of your transaction + change conditions. For example, you concluded transaction with your bank. You created transaction in the system. You settled it. Then you thought that your agreement is quite good. So - you can roll over your transaction. It doesn't matter whether it is the end of the transaction or the beggining - right after settlement you can roll over transaction at any time.
Premature Settlement - I think it's the same as roll over. You can call the function at any time right after settlement - no matter of the transaction terms