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First off, like usual as it is customary in this forum, you were given WRONG answer.
Second the uprev.exe it meant for UP reving NOT down reving that is using it for bringing older version to newer one.
And finally you can not downgrade between major releases you can do it within same release rev.
I did a ups here. uprev up date revisions and as other says it cannot downgrade.
Though there is a way to a "backdoor". Doing the first part demands access to Allegro 16.x eg rent a licens for a short time.
In a skill code run through the footprint library and select view all, and do a clipfile of each footprint.
In v15.x in skill do setup missing layers such as dfa, and others you need. Then run through the folder for each clipfile do an import and save the footprint.