The two different types of illegal designs are: Those that have been accidentally approved by an admin, and ones that were once legal but are no longer legal due to design guide updates.
For imports that were accidentally approved by an admin, admins will waive the fee for an Ares' Mirror if the player in question is willing to fix the design after approval. Otherwise, in the case of missing a trait or marking, or a marking being out of range/illegal, the design is allowed to stay as-is so long as no edits are made. As soon as the character is rebirthed, or has a trait or marking added, the illegal part of the design will need to be fixed. In the case of somebody having added a trait or marking that the character does not have, an admin will Disable the character until that trait or marking is added to the character with an item, or another trait/marking is adjusted into that trait or marking with a Trait Edit or Chimera Scroll.
Regarding Trait Edits, keep in mind they can only turn a trait or marking into another of their kind, and only at the same rarity or lower. Trait Edits also cannot impact Unique traits/markings. Chimera Scrolls ignore the rarity restriction, as well as the "same kind" rule for mutations, but not for regular traits. Chimera Scrolls also can't change any trait into a Unique, if the trait is not already a Unique.
For imports that used to be legal and are no longer because of updates, it will depend on the design guide updates. If a marking becomes out of range, when the character is next changed, you will simply need to pull the marking(s) back into range. If the change is large enough, an admin will waive a Trait Edit or trait/marking application to change the trait/marking into something else or remove it entirely. If a trait or marking has been removed from the game, then admins will adjust the geno to add traits or markings to make the character legal, if possible.
If using a Legacy, Glam Kit, or changing the build of the import (ie, volus to lien), you will need to fix any illegalities. If using Legacy, we are somewhat more loose regarding marking ranges, since we expect the posing of the character will be different from the official import.
If you have an old, no-longer-in-use background, please update it when you next update the design of your character. You do not need any special items to update backgrounds, and may update freely at any time.