what are those knobs at the bottom of the FMCs'
They are cursor control selectors for the electronic checklist.
do LCD screens get retrofitted on 744s that were produced pre-2002 or is the retrofit too costly to justify? (Is it even an option anyway....)
Yes over time the CRTs should all be gone the LCD's are much lighter require less power and are more reliable, don't overheat like the CRT is capable off. I'd say most airlines replace them as their spares and useful life run out.