I can confirm that. The graves are "rented" for a period of time (usually 30-40 years) and if the descendents do not pay for an "extention period", the grave will be levelled.
Sad side effect:
The "rental time" counts from the first reantal date. No matter if someone is burried there or not. If someone wants to reserve a grave during his live time (e.g. for him/her and the spouse) the time is already ticking from that moment.
Especially in case of a double/couple graves this might get complicate if the spouse dies decades after the partner.
PS: needless to mention that the grave "fee" and the maximum duration time depends on the size of the town, the grave yard and the number of available graves.