Processus dans Microsoft 365 pour configurer des applications Office, entrer des clés de produit et activer des licences.
When you say "the Office key modification process is not working." does that mean
#1 - you found a license and removed it?
#2 - Was Office '2019 previously installed on this computer? (and if yes, was it uninstalled)
#3 - Since you mentioned that you have '2019 and '2019 on the same MS Account, what happens when you click on the <Change License> button
Also not clear about when you mentioned that you entered the product key again since a product key can only be entered once and can never be used again.
#4 - Do both '2016 FPE and '2019 FPE appear on your account under Services/Subscriptions?
https://account.microsoft.com/services/
#5 - Assuming the answer to #4 is yes, then if you uninstall all copies of Office on this computer using the Microsoft Uninstall Support Tool (using option #2)
Désinstaller Office d’un PC
https://support.microsoft.com/fr-fr/office/unin...
And then log into your Microsoft Account to install '2016 from Services/Subscriptions (item #4) - does the problem still remain?