Changing an existing preference center form

Sandy put it very well. The actual nitty gritty of an email preferences center certainly depends on the organization, what is priority for them, what isn’t, what their legal team advises based on how their privacy policy/terms are worded, etc. Certainly assuming it’d be okay to send all types of communication to people who indeed opted in for a specific type of communication doesn’t sound good, and you should certainly make sure that you have thought about that case well, and have apt setup in your preferences center to manage it. Additionally, along with the datetime fields, I’ve also generally used the history fields to store the date time history of a person’s preferences change in concatenated manner, allowing to effectively track their older preference updates if required since the CDV activity is kept only for a period of 90 days in the activity log from the activity date.

There isn't a single best practice for migrating/expanding because people store preferences in different ways to begin with. For example, I like to at minimum use DateTime fields, not Boolean fields, because they store richer information. (They tell you when somebody set a preference/subscription and that they they did.) In some cases a Custom Object is even richer. 

If you're only using Booleans and intend to keep doing so, your choices are limited. You might decide that people haven't explicitly opted out of a new preference, so you can start them with it set to true. On the other hand, this isn't ethical if you've previously said they'll only get certain types of content, so you'd start them at false and try to upsell the new subscription options in existing mailings and on your site.

Ultimately, these are a combo of business, ethics, and regulatory decisions and it wouldn't be prudent to give advice unless you can provide more detail (and maybe not even then!)