The Boston Early Music Festival on Wednesday announced cast changes for its upcoming production of Handel’s first opera, “Almira,’’ slated as the centerpiece of this year’s festival. Veronica Cangemi, who had been scheduled to sing the title role, will not perform due to complications with her visa. She will be replaced by Bavarian soprano Ulrike Hofbauer, previously cast in the role of Bellante. That role will now be sung by American soprano Valerie Vinzant. “Almira’’ opens June 9 at the Cutler Majestic Theatre.