As stated, the warm spare method is not a possibility, but I am pretty sure I recall a few years ago having a customer do a similar upgrade and support was able to manually reduce their existing, lesser model license to the appropriate length based on price difference of the higher model. Something like, 3 years left on MX64 changed to 1 year for an MX84. You could call support and ask if that is possible. It would seem fair..
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