How to Migrate from a Classic Theme to a Block Theme Gradually

A classic-to-block-theme migration changes both the frontend design and the way site owners manage templates. The difficult part is not installing the block theme; it is reconciling classic menu locations, widgets, Customizer settings, PHP templates, block templates, Navigation blocks, and Global Styles.

Map the configuration models

Classic theme	Block theme
PHP template files	Block templates and template parts
Customizer settings	Global Styles and theme.json settings
Registered menu locations	Navigation blocks and wp_navigation entities
Widget areas	Block-based template regions or patterns

Build the destination system first

Create the block theme templates, template parts, patterns, typography, colors, spacing, and navigation before routing public content. Treat editor behavior as part of the product: authors need predictable controls and previews during the transition.

Choose a content family with clear templates

A case study or blog post type is often a better first candidate than the home page. It exercises single and archive templates while keeping the highest-risk global and transactional routes unchanged.

Plan navigation explicitly

Do not assume a classic menu automatically becomes a block Navigation entity with identical metadata and behavior. Back up the database, map the source and destination, synchronize deliberately, and validate desktop and mobile interaction.

Consolidate at the end

When the block theme covers the intended routes, move remaining global templates, remove transitional assignments, document editor workflows, and decide which classic configuration can be retired.