Are users forced to choose Plan A or Plan B or can they register without either one? If so you could just add $25 to both of them.
You could also try the following, but in this case users could easily just not pick A. To avoid this do not show Plan B and Plan C at registration and only allow Plan A at registration.
Plan A - $25/single Lifetime, Exclusive, no-Upgrade, Default
Plan B - Exclusive, Upgrade, non-Registration
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--Enhance 1
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-- Enhance 2
Plan C - Exclusive, Upgrade, non-Registration
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-- Enhance 1
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-- Enhance 2