Hearns went up to 160 to fight Murray Sutherland and than went back down to 154 to defend his super welterweight title against Luigi Minchillo. SRL -went up 154 (taking a huge risk on missing his Thomas Hearns mega fight) against Ayub Kalule and than went back to 147 for his classic unification fight with Hearns
The first guy I thought of was welterweight kingpin, Jose Napoles. He went up to middleweight to challenge Carlos Monzon who stopped him in six rounds. Mantequilla then went back down to welterweight and defended his belt four more times. Also Carlos Zarate, the bantamweight belt holder, moved up to 122 but was throttled by Wilfredo Gomez who scored a fifth round kayo. Zarate then went back down and had a final successful defense at 118.
First guy that comes to mind is Bob Foster who went up to HW twice when he challenged Frazier and Ali. After losing both times to them he resumed back to defending his titles at 175.
O'Shaquie Foster, Canelo, Emanuel Navarrete, Orlando Canizales, Marco Huck, Anselmo Moreno, Srisaket Sor Rungvisai. All of them was world champions and during their reigns they moved up during 1 fight and then be back in their categories for defend their world belts again.