This is my understanding of it. A REA message can be sent 30 mins or less before EOBT so if you are ready before that then you either have to wait till 30 mins before and send a REA or cancel and refile. I think it is also the case that no improvement to a slot will be given 15mins before the FPL EOBT although i may be wrong on that.
A slot improvement message is an offer sent by the CFMU and is subject to a time limit, after which if the improvement is not accepted the offer is automatically rejected and the original slot still stands.