Built a custom lightning record based on the code here (the "Making a Record page with collapsing regions" one). It shows up under Custom when i go to build a new lightning record page but when i go to transition an old lightning record page, it is not listed. An another page (as well as the above link) mentions "The order that you list the regions in a page template is the order that the regions appear in when admins migrate region content using the template switching wizard in the Lightning App Builder" so clearly switching existing record pages to a custom template is a functionality that exists. Any idea why my custom template not showing up in the template switching wizard?
It may just take time for it to show up in the template list. I tested with a new custom template a few days ago, and it showed up immediately for new Lightning record pages, but it did not show as an option for migrating an existing Lightning record page. Fast-forward to today, and my custom template does show as an option under Custom Templates for migrating existing Lightning record pages.
For what it's worth, this page specifically states that custom templates are available for new pages.
Once implemented, your custom template is available in the Lightning App Builder’s new page wizard for your page creators to use.
Seems like conflicting (or at least confusing) documentation.