How do I change a Meta campaign from ABO to CBO, or from CBO to ABO, without recreating it?

In Meta Ads (side menu), in the campaigns view, look at the Budget column: next to the budget there is a switch icon. Click it.

• Switching to CBO (campaign budget): you set the campaign's daily budget, and the sum of the ad set budgets is suggested by default.
• Switching to ABO (ad set budget): you set the budget for each ad set, and an equal split is suggested by default.

Nothing needs to be recreated. Meta gives two warnings: it allows one switch every 2 hours per campaign, and the switch may restart the campaign's learning phase.