You will struggle to find "old" graves because in Belgium, graves are not preserved for eternity. "Buying" a plot is not possible anymore. That is the result of a lot of historical events in our area, with successive governments (Austrian, French, Dutch, Belgian) passing laws that led to the current situation. At one point it was possible to buy a "perpetual" concession, which of course was only affordable for the wealthy (abolished as late as 1971). Currently, a grave is kept for a maximum of 50 years. At the time of burial there is a minimum period of, for example, 25 years, depending on the municipality the graveyard is in, after which additional period(s) can be purchased to preserve the grave longer, but never "for ever". After that, the grave, the stone, and therefore also the mortal remains are removed and reburied. Only stone monuments that have a historical or artistic value are to be preserved. It was not until the 18th century that the first voices were heard to break the centuries-old link between church and cemetery. (The RC church refused to bury protestant "heretics" or buried the corpses in a corner, they also refused to bury people who committed suicide, they charged at will for burial, ceremony and mass, made as distinction between paupers and the rich(er) etc.) The eventual transition to secularized cemeteries was very gradual and sometimes not without a struggle. It is in our days the municipalities that manage their cemeteries; not the parishes. In fact, due to the density of the population, there is very little space these days to build new cemeteries and, moreover, an increasing majority of Belgians opt for cremation. A cemetery has a neutral character, and it is not allowed to make a distinction because of the faith or worship of the deceased or the circumstances leading to her/his death. The most recent law regulating the use of cemeteries and funeral services are a regional affair, thus there are decrees valid in Flanders, Wallony and Brussels. If you can read Dutch: here is the decree valid in the Flemish Community:
https://codex.vlaanderen.be/Portals/Codex/documenten/1012053.html#:~:text=Dit%20decreet%20regelt%20een%20gewestaangelegenheid.&text=Iedere%20gemeente%20moet%20over%20ten,een%20gemeenschappelijke%20begraafplaats%20te%20beschikken.