Skill Focus feats and +2/+2 skill bonus feats grant permanent class skill status to the selected skills, in addition to the normal bonuses.

Permanent increases in intelligence (such as from gaining levels or reading Tomes) retroactively grant skill points.

Once a class skill, always a class skill; when a cross-class skill becomes a class skill, the number of ranks in that skill are doubled. (Remember that cross-class skills cost double. This amounts to retroactively treating that skill as a class skill.)

For determining skill points, the class with the highest number of skill points is assumed to be the class that was picked at first level, granting skill points retroactively.

I like retroactive changes because they make for easier bookkeeping and allow more freedom in character creation.