Get ready for a little Cuban chemistry with Grupo Nostalgia. The Latin jam ensemble that won New Times's award for Best Latin Band in 1996 and 1998 has completely transformed itself. The nucleus remains bassist and band director Omar Hernandez. Circling around him for the past few years have been conguero Eduardo Rodriguez, guitarist and singer-songwriter Heriberto Rey, and dazzling pianist Michel Diaz. Long-time frontman and scalawag Luis Bofil split from the group, as did portly percussionist Eddy "Conga" Jimenez. Despite these losses the effervescent crew doubled in size. Café Nostalgia owner Pepe Horta's swell new digs on Miami Beach served as a catalyst to permanently bind the band with old friends who sat in frequently during the Little Havana days. Mellow bassist Mandy Gonzalez, reedlike flautist Mercedes Aval, and vibrant vocalist Viviana Pintado, all of whom used to drop in after gigs with Albita, are now fixtures on the more expansive stage. Saxophonist Juan "Petaca" Silveira and vocalist-percussionist Angel Arce, one of the "Patuti Twins," defected to the group from Manolín's outfit on the Cuban salsero's first trip to Miami. The biggest change of all was brought about by the two new singers. Dashing Nelson Trebejo, formerly with Orchestra Reve, whips the crowd into a frenzy while the sweet-faced newcomer Monica Sierra croons ballads full of soul. Grupo Nostalgia proves the rule: The more dramatic the chemical transformation, the hotter the reaction.