.Mac trial accounts and e-mail-only accounts can be converted to a Family Pack sub-account right away without problem under Account → Manage Accounts → Convert an Account, as explained in the .Mac help (trial, email-only). It’s less clear about converting an existing normal account, though.
The key is to turn off automatic renewal (Account → Credit Card Info → Auto-Renew), let the account expire and then convert it (cf. Converting an expired .Mac account to a Family Pack sub-account.) Apparently1, .Mac will keep all your account data for two weeks after expiration, so if you convert it within this period, you won’t lose anything.