I believe that one reason why clubs are keen to change their badges is copyright. Historically many clubs haven't copyrighted their brand, and that allows firms to make merchandise with the badge on which the clubs do not benefit from. By 'launching' a new badge, they make sure it's protected, then anyone who wants to make stuff with the badge on has to pay a license or royalty to the club.
Premier League clubs have had this wrapped up for some time now, 2 years ago I was instructed to remove the premier league team badges from my website unless I had the 'rights' to use them, the cost would be prohibitive so I removed them. I was also told this included older versions of clubs badges which we covered by the modern usage rights.