2 of 4 added clarification

What branch strategies to use for migrating from waterfall to true continuous integration?

In a (very) large organisations using the waterfall methodology typically very complex branching structures (aka branch spagetti) are encountered.

What branching strategies can be used in a transition to true continuous integration - a single main development branch (aka trunk-based development)?

Update:

To clarify, the question is about the migration/transition itself, not about the before and the after methodologies which are pretty clear.

It can't really be "at EOB today we're still waterfall with gazillion of branches but tomorrow 1st thing we'll switchover to trunk-based, single-branch CI".