Now I need to do this and waste a lot of hours. 
I really hope one day this option will be available.
Topic summary
Main issue: Upgrading Shopify’s Dawn theme from version 12 to 15.0.2 without losing custom code.
- Consensus: There is currently no automatic tool or quick method to migrate customizations between Dawn versions. Custom code must be manually re-implemented in the new theme.
- Precaution: Back up the existing theme before updating. An image was shared showing the backup step, but it’s not essential to understand the guidance.
- Best practices: Maintain detailed documentation of every customization (what changed, where, and why). This significantly shortens re-implementation time when upgrading.
- Experience shared: With excellent documentation, re-customizing a full site after an update can be completed in about a day; without it, it may take many hours.
- Status/outcome: No automated solution; recommended approach is backup + thorough documentation + manual re-application of changes. The original poster remains frustrated and hopes for a future migration option.