It means that Lifeimi Mafi has been promoted from the bench to take Howlett's right wing, while Denis Hurley will take over on the other wing from Dowling.
Cork Constitution clubman Tom Gleeson, who is yet to make his Heineken Cup debut, has been added to Munster's replacements bench.
Munster have won four previous semi-finals in the Heineken Cup and famously beat Biarritz Olympique in the 2006 decider in Cardiff.
Their captain for Sunday's game, out-half Ronan O'Gara, insists that the province's previous record will not count for anything in northern Spain.
"We have been here before but that's in the past. The great thing about sport is that it happens in the present," he said.
"We are experienced in big atmospheres and games like these are why we play the game. The feeling of winning big games is very hard to talk about, especially what happens in the game or what happens in the dressing room.
MUNSTER (v Biarritz Olympique): Paul Warwick; Lifeimi Mafi, Keith Earls, Jean de Villiers, Denis Hurley; Ronan O'Gara (capt), Tomas O'Leary; Marcus Horan, Jerry Flannery, John Hayes, Donncha O'Callaghan, Mick O'Driscoll, Alan Quinlan, David Wallace, James Coughlan.
Replacements: Damien Varley, Julien Brugnaut, Tony Buckley, Nick Williams, Niall Ronan, Peter Stringer, Tom Gleeson, Scott Deasy.