Hmm... if you are going for just one change, adding a semicolon after the word UFO would be the best choice. I would add an additional comma after the word however though, if you can make more than one change.

Everyone at the convention claimed to have seen a UFO; however, no one had taken a picture of one.

In the Night Sky I Recall a UFO - 2013 was released on:

USA: February 2013

USA: 5 February 2013 (Rosendale, New York) (premiere)

USA: 1 March 2013 (International UFO Congress Convention Film Festival)

Because they may not appreciate being surveyed and they may not want to be surveyed.

2012 We're Already in It - 2009 was released on:

USA: 26 March 2009 (International UFO Congress Convention Film Festival)

a UFO See http://grammar.ccc.commnet.edu/grammar/abbreviations.htm (all the way to the bottom) for help determining when to use a or an before an abbreviation.