Possibly helpful, from the horse's mouth (amongst other FAQs):
I am down as a First Officer, what will happen if my ATPL get unfrozen before I start on the line? Once you have a full ATPL you will be eligible to become a Senior First Officer. If you get your ATPL licence before your course starts you need to send a copy of it to the pilot recruitment department who will then send you a change of rank letter. If this happens during the course of your training you need to bring the copy to the training department.
Anyway, if you are definitely soon to join EZY (as opposed to applying) then why not ask them directly, rather than us random collection of weirdos on PPRuNe? And why not worry about these things nearer the time? For now, just relax and enjoy the line training!