If you upgrade from JUCE 7 to JUCE 8, with the 30% discount, then your JUCE 8 licence will replace your JUCE 7 licence and you will be bound by the JUCE 8 terms. If you purchase an additional JUCE 8 licence, without the discount, then you can use your JUCE 8 licence for JUCE 8 projects and your JUCE 7 licence for JUCE 7 projects.
Contact sales@juce.com. In the short term we can make some exceptions.