As an option in case like me, you don't want to start a new seed every major content drop and just want to keep your current seed... There is a mod called Upgrade_World that you can use to upgrade your current seed with the new things. I've used this twice now, once to add tar pits and once to add mountain caves. Can confirm it works very well!

Be sure to backup your world files before running the upgrade just in case something goes bad!