The answer is it depends.
Catalyst slide-valves are not only used for control but are safety devices.
Restarting the FCCU/RFCCU with a by-passing/leaking slide-valve depends on if there
are other means of stopping catalyst circulation and sealing the catlyst transfer lines,
isolating the Reactor and Regenerator (such as slumping the catalyst).
IF there are no secondary means of stopping catalyst flow and isolating the Reactor
and Regenerator vessels; then the answer from a safety point of view is NO, one should
NOT restrat the FCCU/RFCCU!