I've got a couple of flights booked in various classes and don't understand the significance of the class codes. Can anyone explain and is there any way to tell what they'll be before completing a booking? Are there significant differences between them? I understand the basic differences but are there more specific things to watch out for with Emirates?

My examples are:
1) Flights both inbound and outbound are "Q" which is normally a discounted fare but it's being treated as a Flex fare when I try to upgrade, although it's ticketed as a Saver.
2) Flights are "B" outbound and "K" inbound (again I understand "K" is a discounted fare). This is ticketed as Flex fare despite the discounted legs.

Any thoughts or clarification appreciated.

In general, booking classes in the same cabin mostly serve as a way to differentiate inventory and, as you experienced, generally tell you very little about the restrictiveness/changeability of your purchased fare. On some carriers, even Y fares can carry refund restrictions so there isn't one steadfast rule. The only way to know for sure is to look at the fare rules before you purchase your ticket.