FCLoopSwitchEtaSign[exp, s] switches the sign of i \eta in all integrals to s, where s can be +1 or -1.
Notice to change the sign of i \eta the function pulls out a factor -1 from the propagator.
Overview, FCTopology, FCLoopGetEtaSigns.
FADs are automatically converted to SFADs, since otherwise their i \eta prescription cannot be modified
FAD[{p, m}]
FCLoopSwitchEtaSign[%, 1]\frac{1}{p^2-m^2}
\frac{1}{(p^2-m^2+i \eta )}
FAD[{p, m}]
FCLoopSwitchEtaSign[%, -1]\frac{1}{p^2-m^2}
-\frac{1}{(-p^2+m^2-i \eta )}
SFAD[{p, m^2}]
FCLoopSwitchEtaSign[%, -1]\frac{1}{(p^2-m^2+i \eta )}
-\frac{1}{(-p^2+m^2-i \eta )}
CFAD[{p, m^2}]
FCLoopSwitchEtaSign[%, 1]\frac{1}{(p^2+m^2-i \eta )}
-\frac{1}{(-p^2-m^2+i \eta )}