I'd like to add some homebrew content to an already-running campaign, but I don't see any option to manually enable this. Do I have to create a whole new campaign to replace the current one with the different settings?

All homebrew in your homebrew collection is automatically shared within DDB campaigns. In fact, all homebrew in everyone’s collections are automatically shared within campaigns. Please see the General Homebrew FAQ #5 for more information on how to enable it for each character: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).

I've tested this and it doesn't seem to be the case. I whipped up a quick warhammer, and I can add it to my inventory in another campaign I'm in. But in the campaign where I'm the DM, it doesn't appear in the search results when trying to add it to a character's inventory.

Okay, I found the issue. Homebrew needs to be enabled per character it seems, and I never realized this. Found the answer in your FAQ, so thank you!

I’m glad! If you don’t mind, please vote in the survey attached to that FAQ and hit the like/thank/upvote button for it too if you haven’t already. I’d appreciate it, thanks!

Done!

Thanks!!