Zorba,
Firstly, SRC tickets must be issued within one year of leaving, and utilized within one year following issue date. There seems to be some flexibility in changing destination within the ticketing zones after the ticket has been issued, and each ticket can comprise 6 sectors - eg, if you live in UK after leaving, your ticket can be LHR/DXB/SIN/MEL/SIN/DXB/LHR. As I understand it, the same ticket could be used BHX/DXB/KUL/MEL/SIN/DXB/LHR, for example.
My experience is that you simply have to plan ahead. Don't expect to be able to decide two days in advance that you want to travel using SRC Firm tickets and expect to get seats on your desired flights. Seat availability for SRC Firm tickets changes on a daily basis, and sometimes more frequently, so you have to keep watching on Trips, which you will continue to have access to after leaving after 15 years of service. The other consideration is that you can't get these tickets online - you apply for them by email to Staff Travel, and you have to arrange to pay for them (taxes), and get them issued at your local EK office. The response time from Staff Travel varies!
Having said all that, it does work if you plan ahead appropriately - my wife and I have just flown KUL/DXB/MAN and BHX/DXB/KUL on SRC Firm tickets which we booked a little in advance, and successfully got First on sectors where the cabin was available.
You need to be aware that SRC tickets cease to accumulate after leaving and Retirement tickets kick-in, though I think the rules are the same.
Hope this helps. PM me if you have any questions which I might be able to help with, but its all spelled out in detail in the Staff Travel Manual.
7B