Welcome to Blue Ribbon Blues!
In 1943, Leon Schlesinger wanted to keep up his 39 reel contract and came up with the "Blue Ribbon Hit Parade", in which the original cartoons' opening sequences were spliced out and replaced with new titles that didn't credit anybody, and sometimes entire scenes were removed. Later on, they seemed to learn from their mistakes, and starting with Mouse Mazurka (1949), the original opening titles were kept. This site was created as a helpful guide for this subject.
UPDATE - 2021-08-01: Minor update on the Checklist.
The Title Card Gallery - Checklist - Contact Page
LEGAL DISCLAIMER - Looney Tunes, Merrie Melodies, and all related logos, characters, and symbols are the property of Warner Bros. Animation, Inc. I do not own the legal rights to any of these images, I am just a fan with a bit of HTML knowledge.