It depends on what you mean by "retirement orbit." Some satellites are left on-orbit but not used much, when their transponder complement (and the revenue it can produce) no longer justifies taking up the orbit slot. At this point, the transponders are turned off, but the satellite is left functional--it may be kept as an on-orbit spare, to be put back in service if another satellite fails. Although the transponders are off, the satellites are not--their power and attitude control systems are left on. So if this happened to our "host", the amateur payload could continue to operate. These satellites don't typically drift--they're "parked" in an orbital slot but just not actively used.
If by "retirement orbit" you mean "disposal orbit" (typically about 300 miles above GEO), satellites are typically not put there until they are not worth anything to the owner. This is most often due to on-board failures or running out of expendables (e.g. fuel) that prevent the satellite from ever continuing its mission. At this point, the owners do not want to spend any time/effort/resources continuing to maintain them. As a result, their propulsion systems are vented (to mitigate future explosions) and electronics are turned off and disabled in a way that prevents even accidental turn-on. This would include the attitude control subsystem, which would keep the spacecraft pointed at the earth, as well as the electical power subsystem. A satellite in this condition could not continue to support any other payload.
(Note that a GEO satellite is registered to operate from a single orbital slot, but satellites in the "retirement orbit" are drifting. They would cause interference to other registered satellites if their transponders were to remain operational during that drift. So to prevent retired-and-drifting satellites from unintentionally causing interference, operators will when possible deliberately disable a retired satellite.)
Of course, such a drifting transponder would continue to be useful to the amateur community. This is something that could be a subject of negotiation between AMSAT and the host.
73, Steve Melachrinos W3HF
Does anyone know if retired birds are switched ON or OFF once they get boosted into a retirement orbit ? Could still be useful to us even from there.
73 Richard Limebear G3RWL