What happens when I change an item's template?
The data from custom tabs and every other tab will be retained.
Let's say you have two templates: Template A and Template B.
Template A has a custom tab with custom fields.
Template B only has the default tabs.
If Template A is filled with content, from default tabs to custom tabs, all data associated with it will be retained if you switch to Template B. That means should you decide to switch back to Template A, you'll see the contents of the custom tab safe and intact.
However, sending an item back to brief will delete all data it contains, except for some text in the main editor in the Text tab.