Hi Oberon 1, great handle by the way. Did you used to be king of the fairies?!
I would see if you can get hold of another company's CAMME (Continuing Airworthiness Maintenance and Management Exposition). Ours are now held on CD and replaced regularly by amendment.
In it you can see what tasks are available for each station. For instance, some stations may be considered as main base, so a scheduled engine change could fall under their remit. Some other stations may be considered as outstations so just basic tasks up to weekly and defect rectification might be their limit. Your example of an engine change is good one, as some line stations may not be permitted to do a scheduled one, but could if it was an AOG situation.
These limitations are set by the Quality department and are ratified by various factors such as number of certifying staff, their qualifications, facilities available on station etc.