The "upgrade licenses if needed" statement is probably there for the following reasons:
1) In the old days anytime there was a "major" version upgrade (i.e. R4.X->R5X, R5X->R6X, R6X->R7X, etc.) the license generation process/scheme would be updated and the licenses would need to be regenerated for the new major version. Mercifully, the bladed licensing scheme did not change R7X->R8X.
2) There are many features/blades that are separately licensed, yet always included with every firewall license sold. Examples of these would be Content Awareness starting in R80.10 and Identity Awareness starting in R75. However if you are still using a license generated against an older version prior to these features being added, the required licenses for these new features won't be present and you won't be allowed to use them after the upgrade.
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