Testnet and mainnet on Ceramic are two different networks (similar to Ropsten and Ethereum mainnet) for example. Once we launch mainnet we will launch from scratch, the user profiles will have to be re-created, along with the following/followers, groups etc. We will also be using schemas everywhere, which we weren't doing before.

Mainnet will also be using a modern decentralized id (‘did’) system, so your did won't be the same on mainnet as it is on testnet and we can't have automatic migration between the two.

There isn't going to be any loss of content though, the testnet app is always going to be available and users can decide to keep on using it if they want to or to go back to it sometimes to read the content of previous AMAs etc! It will live on a different subdomain.