Migration guide should be more elaborate on the breaking changes and migration of Grid about flow-and-components-documentation HOT 1 OPEN

The limitations for HeaderRow and FooterRow compared to V8 should be mentioned. Because the headers and footers are linked to the client-side column grouping concept,

• Cells can be joined only on the currently out-most layer
• Prepended header rows and appended footer rows have the same cell structure as the previous row (same cells are joined)
• The header and footer structures limit each other. For example, if you join cells 1 and 2 in a header row, you can't join cells 2 and 3 in a footer row (without joining cell 1 also), because it's not possible to create a hierarchical structure of <vaadin-grid-column-group>s like this.

from flow-and-components-documentation.