Teasers don't usually have credits regardless of when it appears. In the first six seasons Ironside began with the theme music, then a credit-less teaser and then act 1 with the title and credits. I see no reason to suspect that the Bonanza openings were changed.
Why would they change it for reruns? Series change formats all the time. (The Wild Wild West pilot also had the theme first.) Was season 14 ever in syndication? Syndication Is programming sold to local stations not necessarily reruns. I thought season 14 was part of the "lost" episodes. These...