Minetest doesn't recalculate existing zones.
If you change something about map generation (new trees or different settings in map_meta.txt), it does not change anything in already existing areas, which have already been generated. But if you go to an unexplored place (an area which doesn't exist yet in database), it will be immediately generated, and the new parameters will take effect. So if you change mapgen params, you may get some discontinuities between new and old terrain.
If you update to the latest version, you will see new biomes and plants appearing at new places. That's the only change that could create discontinuities, because biome data aren't stored in map_meta.txt.
Some default parameters have been modified, and will be effective for new worlds but map_meta of old worlds will not be changed at all.
Very busy this year too, so do not expect me to be very active on the forum or in game. But I'm not about to drop Minetest forever :)