Kerry admitted as much to the Boston GLobe
http://www.boston.com/globe/nation/packages/kerry/061603.shtml
Kerry initially hoped to continue his service at a relatively safe distance from most fighting, securing an assignment as "swift boat" skipper. While the 50-foot swift boats cruised the Vietnamese coast a little closer to the action than the Gridley had come, they were still considered relatively safe.
"I didn't really want to get involved in the war," Kerry said in a little-noticed contribution to a book of Vietnam reminiscences published in 1986. "When I signed up for the swift boats, they had very little to do with the war. They were engaged in coastal patrolling and that's what I thought I was going to be doing."
I don't think that the swift boat assignment changed two weeks before Kerry arrived, though. I think that operation Market time was initiated in February of 1968, in response to the Tet Offensive. My husband had two swift boat crews assigned to be based on his ship at that time, and I think I read this, straight from his Vietnam year book.
In an intense three months of combat following that Christmas Eve battle, Kerry often would go beyond his Navy orders and beach his boat, in one case chasing and killing a teenage Viet Cong enemy who wore only a loin cloth and carried a rocket launcher. Kerry's aggressiveness in combat caused a commanding officer to wonder whether he should be given a medal or court-martialed.