Users selecting the Reader Mode template mode are guided through the selection of the theme to be used:

Existing users of Reader mode using legacy post templates

• Continue using the Legacy theme as the default when they upgrade the AMP plugin.
• Provide ability to switch from legacy post templates to full site reader mode theme.

New users or existing users enabling Reader mode

• Choose a theme from a curated list of AMP-compatible themes when switching to Reader Mode (or already in Reader mode)
• Not allowed to select the currently-active theme as the theme for Reader mode since such a case corresponds to setting the AMP plugin to Transitional Mode.
• If a user changes the active theme at some point to be the same as the theme used for Reader mode, then they automatically get switched to Transitional mode.
• Users may still select the legacy post templates, but this will be clearly marked as legacy/deprecated.