Best Practice Guidelines
Do as much as necessary, but as little as possible.
Slight leans are part of the character of old cemeteries – leave them alone.
Moderate and severe leans need to be corrected, to prevent damage due to gravity and their attraction to vandals.
The degree of subsidence below the foundation needs to be addressed.
The accepted method is to provide a foundation of substance, usually by placing a plug of concrete in the subsided area, replace the stones. Sometimes it will be possible to lever the headstone upright in situ, hold, and plug foundation with concrete, then lower.