Summary
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A member can upgrade to higher product
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A member can apply promotion code at mypage.
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A member can delete their account at mypage.
How upgrade works
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Upgrade works for members who are 'subscribers'. People who bough a product with 'onetime' type of Price, can not make an upgrade.
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A member who are subscribing a lower product see a 'Upgrade' button rather than 'Subscribe' button when they try to get access the page with permission of higher product.