When determining if a monster’s effect should be negated by “Light-Imprisoning Mirror”, check the monster’s Attribute at the time its effect is resolved.
When a Non-LIGHT monster’s effect is activated while the monster and “Light-Imprisoning Mirror” are both face-up on the field, if you chain “DNA Transplant” and select “LIGHT”, the monster’s effect will be negated if the monster is still face-up on the field when its effect resolves because it is now a LIGHT monster.
“Light-Imprisoning Mirror” cannot negate the Continuous Effects of LIGHT monsters.