Ive been pondering this topic myself, trying to decide what to do in my campaign. I know how it works in D&D (a skill is a class skill only for the levels and skill points its "actually" a class skill in your list) but in T20 with prior history its just soooo much easier to say "once a class skill, always a class skill". Depending on your campaign, the before mentioned approach could be a license for min/max people to make some obscene characters. Ive already seen a few.
Does it state in the T20 book anywhere how it works?
I kinda like all my PCs to be well rounded in the skill department. Since its so deadly, redundancy is a good thing. All in all I guess im saying its your world, so do what feels right.