2004-10-04
Its power/toughness changes when the cumulative upkeep resolves, not when it triggers.
2008-04-01
If you choose not to pay Mwonvuli Ooze's cumulative upkeep, it will still get an age counter before being sacrificed. Its power and toughness will momentarily increase before being put into the graveyard, which might matter to certain other cards (such as Proper Burial).