you can keep playing your world for good aslong as you never explore too much of the whole map. its very easy to avoid ashlands and deep north areas by simply never going all the way directly north or south to the edge of the map.

new enemy spawns happen regardless of how old a world is as enemys are spawned in dynamically.

so just explore into east or west in a huge cone and youll get everything while also leaveing tons of unexplored map for later.

they wont make THAT many additions to existing biomes anymore at this point probaly. the most important part is to not visit ashlands or deep north too much. as already explored areas of a map will not update anymore even after the update since they already got generated.

only way around that is to cheat with mods. but that can have some risk to it depending on world layout and bases/spawns.

No you can use the old world.

In addition to the excellent advice above their is "Upgrade world" a mode you need only use once then uninstall and play vanilla on the refreshed version of your old world. Been using it since game launch on one worl over 1500 days old now and has all new content

Originally posted by Qubo Night Owl:

Do you need to start a new world for it to have the various things they have added to the game since the initial early access release.

Only if you have already explored the biomes that were added since you played; if it's already discovered, it doesn't (re)generate properly.

That being said (and assuming you need to make a new world), you can use your character to snag your gear and any materials you've already farmed through the magic of world-hopping. Log into your old world, grab anything that isn't nailed down that fits into your inventory, then log out. Create a new world, log into it with the same character, build chests, and dump your inventory. Then log out, log back into the old world, grab more stuff... lather, rinse, repeat until you have everything you want/need to start out as strong as you were when you last played.

Originally posted by Qubo Night Owl:

Do you need to start a new world for it to have the various things they have added to the game since the initial early access release.

if there is an unexplored area on the map, then new islands will regenerate in a new way (with new things/mobs/dungeons). I play solo on the map, the plains were scouted, then there is life on them, but I met new things on new islands, also mistlands lands (the scouted island has a "broken" appearance), but after scouting the new island after the update, everything is ok(at new).