Why were they slightly different than the opening credits for the 1965-1970 seasons?By this,I mean in the earlier seasons,they just did the opening credits and the whole episode followed.In the 1970-1971 season,however,the episode started with part of the episode and then the opening credits.Why was this?

Can't think of an example, but I have seen that with some other shows as well, where for part of their run they would do a teaser before the opening credits.

Teasers seem to be much more common on sitcoms (and other shows) in general today, if the shows have an opening theme at all. I recall this about Green Acres Season 6 as well. The show would open with a brief shot of the barn with the show title painted on it, with maybe a few notes of the theme song, then have a short teaser, then go into the opening theme song. Shows like Gilligan's Island, The Brady Bunch, and The Partridge Family had teaser scenes after the opening credits and before the first commercial. The Partridge Family was a bit different from the other two productions in that the creative credits (Exec. Producer, Writer, Director) were shown after the opening commercial over the show logo instead of over the action of the teaser as the other 2 shows did.