Your question is based on the assumption that there was anything wrong with using upgraden as a German verb. I disagree with that. It follows a very common pattern of using foreign-language verbs in German: use the foreign stem, add a German verb ending and apply a German conjugation pattern. Amongst tons of other examples compare for instance
- probieren (from Latin probare)
- promenieren (from French (se) promener)
While verbs from romanic languages often get the suffix -ieren to replace the romanic suffix, verbs loaned from English are regularly used with the -en suffix. Compare for instance
- skypen (from the brand name of the voice-over-ip-service Skype) as in Wann können wir mal wieder skypen?
- framen (from English to frame) as in Es gefällt mir gar nicht, wie Sie diesen Text framen.
- downloaden (from English to download) as in Ich habe mir das Programm downgeloadet
- grinden (from English to grind) as in Grinden ist ein Fachausdruck aus der Skateboard-Szene.
- adden (from English to add) as in Ich habe dich als Freund auf Facebook geaddet.
- liken (from English _to like) as in Schon 100 Leute haben mein neues Profilbild geliket!
So, I would say the perfect way to express this sentence is indeed
Wenn der Kunde auf 7.2 upgradet, braucht er eine neue Lizenz.
Most of the verbs which have been proposed such as hochgehen, aktualisieren, aufrüsten are not common and sound more strange than upgraden. Also the attempts to replace the verb upgraden by the noun Upgrade and some semantically weak German verb like durchführen are not common and sound rather bureaucratic.