Clarify Identity and data migration during upgrades

[zaycodes]

Description

The Identity and data migration sections in the upgrade documentation are currently difficult to follow, especially for users who are less familiar with the internal architecture.

During review, this was called out as an area where the flow and explanation could be improved.

This issue is to explore ways to simplify and clarify:

• how Identity and data migration work during an upgrade,
• what happens automatically versus what requires user action, and
• how these migrations relate to the overall upgrade process.

Potential approaches may include restructuring the content, adding visual explanations (for example, diagrams), or improving the narrative flow, but the exact solution should be evaluated as part of this work.

Outcome

Users should be able to understand, at a high level, what is being migrated and why, without needing to infer behavior from multiple pages.