When the stock design updates
Platform updates sometimes improve the stock designs: a topic row gains the labels, a profile card gets a new detail. If you've customized that surface, here's the promise this whole system is built on: your template keeps rendering, untouched. Nothing is ever overwritten by an update.
What you'll see instead
The Custom Templates page shows a friendly banner: a template needs review. The surface gets an Outdated chip next to its Customized one, with a Review changes button. Outdated doesn't mean broken; it means the stock design gained something you might want.
The review
Opening the review shows your template in the editor with a panel beside it: what changed in the stock design, line by line, additions and removals marked. From there you have three honest options:
Merge what you like. Copy the interesting new lines into your template, save, and the review clears itself.
Keep yours exactly as it is. Press Mark as up to date; your template is untouched and the reminder goes away.
Take the new stock. Revert to stock adopts the updated design entirely. Your old version stays restorable, like every save.
There's no deadline on any of this. An Outdated chip can sit there for a month while your board renders your design happily. Review when you're curious, not because anything is at risk.