The Public and Commercial Services Union (PCS) has urged Southbank Centre to rethink a restructure of host roles at the Hayward Gallery.
Staff rehired on fixed term contracts after being made redundant last year have been told they will be re-employed under new contracts from August 1.
The contracts specify a pay reduction of 20%, with fewer hours available, according to PCS.
A Southbank Centre spokesperson said the roles no longer include ticketing responsibilities because most tickets are purchased online.
"Our aim is to treat everyone across our site fairly for similar roles. We expect these discussions to conclude in the next few weeks."
The union said it will "fight this attack" by doing "everything we can to challenge these fire and rehire tactics".
"For the Southbank to treat them so poorly in spite of their hard work and dedication during this difficult time [is] a disgrace," a statement from the union added.