Vee,
Re changing / booking seat on aircraft, this is how it has worked for me in the past and worked well I must say.
When you get your final documents you should then have your E TICKET NUMBER, once you have that you can go on the airlines main web page, re seating as in it might say ...Can I book in online, then follow that link, it usually takes you to a seating plan of the flight and shows you the seat they have choosen for you, if thats fine you can still carry on booking in online, then you only have to go to baggage drop off at the airport as you will of printed out your boarding pass from your home computer. IF you do NOT like the seat they have choosen then it gives you an option to PICK A SEAT!!! so you do that and the proceed with booking in online. This is approx 24-36 hours before you go away.
You normally need your E TICKET NUMBER, and PASSPORT to hand when you are doing this. Once you have followed all the steps then you can print out your boarding pass and then when you get to the airport you just need to go to baggage drop of as you allready have your boarding pass.
I normally just locate the JY rep at the airport and say I have booked in online and they point me in the right direction if your worried..OK all the best.
Cindy