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This is a short article in response to Jeff Reints most excellent Dungeons & Dragons article What do Spell Levels Measure? available here.

In the early days of the game it was a commonly held Interpretation of the wizard's spell list that the spells contained within the list was only the beginning. GMs were encouraged to make up their own spells. Unfortunately, like everything else, there were no guidelines for this, and this quickly evolved into a power gaming match up where the players who played Wizards dreamed up new spells, and wrote up the spell descriptions, then introduced these new spells into the game. These spells were so unbalanced that they often broke the game, either by destroying the verisimilitude of the game world, or by providing the player who played the wizard a tremendous advantage. This wrongly helped create the impression that Wizards, especially  at high levels were overpowered.

Spell Progression Variant for Old School Games:

Spells don't automatically do more damage nor do they have a larger area/duration of effect at higher levels... They must first be researched once more, by the Wizard in question, before they produce the enhanced effects that the spell description provides.

Wizards can cast the spell at the newer high level without doing any research with the higher level effects as written, if they make a successful saving throw versus magic or save d20 vs INT. If they fail that saving throw, the spell misfires or backfires.

Spell Backfire Table (roll d20)

01 Backfire, spell does double enhanced damage to spellcaster.
02 Misfire, spell fails utterly.
03 Misfire, spell creates spectacular light show or illusory SFX, but does no damage/effect to target.
04 Misfire, spell creates 10'radius circle of darkness per level of caster centered on the target.
05 Backfire, spell creates 10' radius circle of darkness per caster level, centered on the spellcaster.
06 Misfire, spell does enhanced damage, centered on any random target within 25' of the original target.
07 Misfire, spell creates spectacular audio effects, but delivers no damage or effect upon target. i.e. spell delivers sonic boom, ambient musical effects, bells, chimes, harp sfx etc...
08 Backfire, If no other friendly targets are available in range, spellcaster becomes the unintended recipient of enhanced spell damage/effect.
09 Backfire, spellcaster takes full enhanced damage from spell.
10 Misfire, spell re-targets another creature/character within 20' of intended target.
11 Misfire, spell duration halved.
12 Misfire, spell duration doubled.
13 Misfire, spell only does 1hp of damage per level of the spellcaster.
14 Backfire, spell does different damage type... If fire spell , does ice damage, If magic missile, does non-lethal damage, etc. et. al.
15 Misfire, spell range halved.
16 Misfire, spell range doubled.
17 Misfire, spell strikes inanimate object within 25' of target. Spell does enhanced damage/effect.
18 Misfire, spell effects create an opposite effect... i.e magic missile heals target, healing spell damages target even more, etc.
19 Misfire, spell effects original target with enhanced damage, and also randomly strikes one additional target within 10' of original target, including possibly the original target (If no other targets are available) one round later.
20 Critical! Double Damage/Summoning effects! Original target takes double the enhanced damage.

If the spell is successfully cast, the spellcaster delivers the enhanced damage that the spell provides but only for that casting time. If the same spell is cast again later, the caster is still subject to spell misfires or backfires until such time as the spell is successfully re-researched!

Spell Research

In order to derive permanent benefits of the spell level increase for spells already contained within their spellbooks, Wizards must research the spell again, as if it were a new spell (After all, it is). To research the spell takes one week per spell level, and also costs 50gp per spell level. At the conclusion of the researching of the spell, the caster makes a roll of 1d20 vs. INT  (Player's Note: Roll lower than your intelligence, in order to grok and successfully copy this new spell into your spellbook).

If you succeed in your research roll, give your new spell a new name, copy the relevant details into your spellbook,  and roll d12 to determine the new requirements/effects of the spell.

Wizard's Spell Research Table (roll d12)

01 spell has new verbal or somatic component . Describe while casting.
02 spell has new material component that costs 50 Gp/Lvl. Describe.
03 spell delivers new light colors or luminary effects. Describe.
04 spell delivers new audio or sound effects. Describe.
05 spell delivers different damage type than original. Describe.
06 spell delivers double damage/effect. Describe.
07 spell delivers half damage/Effect. Describe.
08 One type of creature/monster is unaffected by your new spell. GM Determination, but you know it's going to happen!
09 spell actually turns out to be another common spell (Roll randomly from the list of available spells in PHB or Bx book. ignore and re-roll for spells your already know) to determine which new spell you just figured out!   
10 spell automatically selects original target plus the next closest target (friend or foe) and delivers 1/2 damage or duration of effect to each if they fail their Saving Throw.
11 No need to roll damage, enhanced spell automatically does maximum possible damage/effect if target fails the saving throw.
12 Roll 2x on this table.
 
footnote:

In Issue #242 of Dragon Magazine, an article was published, The Laws of Spell Design by Ted Zuvich. This article went a long way towards creating a viable method  for GM's to figure out what level the spell should be, and a reference to that article is included in this article for those of you that haven't seen it so you can arrange to get your eyes on The Laws of Spell Design to grok the gaming goodness contained therein.

verisimilitude: Credibility/Plausibility/Realism  of the fantasy world determined by the players who share a common understanding of how the laws of physics work, how the rules work, and by their shared experiences within that game world/model.