Uller said:
So what if the rules change after you've gained a few levels? What if the DM decides your group is abusing a spell (see my polymorph thread) and changes it a little? What if new feats or spells are introduced to the campaign? Or do you go into a campaign with a certain set of rules and those are fixed for the duration? Some people play short campaigns and that is reasonable. Others play campaigns that last for years and so it is very likely that rules will change as the campaign progresses. If you've made decisions based on a certain set of rules and those rules change, that could be bad.
Example: A new first level spell is introduced after the sorcerer has already learned all the first level spells he'll ever know but the player wants that spell and probably would have chosen it early in his career. Would you let him swap out a little used 1st level spell, or make him burn a feat? I'd probably go with the former (unless the spell was introduced as a newly researched spell by an NPC).
Not trying to argue here...just curious.
Did you miss where I mentioned, at least once, that "the Plan" was subject to revision by me, when and as I felt it needful?
If new rules come out, I look them over, and see if I can change -future- choices to something more favorable to me, using those rules. If not ... oh well ... maybe next campaign.
If the sorceror has already learned "all the firstlevel spells he will know", that means the sorceror is 7thlevel or higher ... and IMO, if the spell is THAT good that I MUST have it, it's probably under-rated WRT it's level

. Otherwise: I can re-assess my feat choices, and pick up the feat from T&B (I forget the name) which permits Sorcerors and Bards to learn a single additional spell.
Or, per the PHB rules, I can simply surrender a less-important HIGHER level spell in favor of the lower-level spell; while the rules do not specifically allow this, I see no reason why it oculd not be done (since you can "undercast" using, for example, a 5thlevle slot to power a 2d level spell ... I see no reason why "under-learning" should be forbidden; in fact for moderate-CHA sorcerors (multiclass perhaps), who might not be ABLE to learn spells of, say, 7th or higher level ... this would almost seem a required allowance).
Either way, past choices, barring in-game effects, remain immutable.
Allowing revision because the RULES themselves change is different than allowing revision because you made a poor choice. If the rules change, allowing the player to make the choice they WOULD have made if the rules had ALWAYS been the way they are post-change, isn't a violation of the spirit of the rules regarding Sorceror spell-known choices.
But allowing the sorceror to say "gee, this spellis kinda useless after all, can I change it for something different?" ... is IMO like the Rogue saying "gee, I shoulda checked for traps BEFORE I got decapitated, can I go back and do so?"