With the introduction of Block Themes in WordPress 5.9, there are a few considerations to take into account when switching block themes or when upgrading from a classic one:

Theme selection

• Browse Mode: Add theme previews #47228

Template and themes decoupling

• Remove coupling of templates, parts, and patterns when switching themes #25071
• Templates: Retain custom templates when switching themes #31954

Navigation

• Preserve the navigation on theme switch #38291
• Automatically utilise classic menu when switching from classic Theme #42600

Styles

• Clear block-level inline styles on theme switch and other actions that affect global styling #21045
• Proposal: Standardized block markup, theme.json design tokens, and CSS classes to improve interoperability #38998
• Standardizing required theme CSS Variables #33180

Classic theme upgrading and backwards compatibility

• Provide a pathway to migrate widgets to block themes #39270
• An equivalent to "Inactive Widgets" for template parts #31397
• FSE themes should have a way to set a default header and footer that maps to traditional header.php and footer.php in terms of backwards compatible loading #33989