I can tell you how it works in the cemetery of Trapani, Sicily.
My family has a graveside forever, nobody can ever take out the old caskets to make room for new members of the family.
The living member of the family can rent a new graveside for 19 years or for 99 years. When the rent is over the manager of Cemetery record office in Trapani will send someone to put a sticker on the tomstone stating that if the family members are still interested to have the rent renewed, they have to contact the Cemetery record office and pay another rent before a certain date. Otherwise the Cemetery record office will afford the costs and expenses to take the casket out so there is another graveside is ready to welcome some other casket.
The old caskets with the bones is put in a common grave.
I hope it helps,
I guess it all depends on the town and how old the graveyard is...as a child (20 yrs ago) when i visited my parents town in molise the graveyard was overturned...i distinctly remember trying to put the bones back together like one big puzzle! i was told then that this was common during the time of war and poverty, the graves were robbed for any valuables.