SQL Server 2014 Edition Downgrade
Event though it says its a downgrade, I would say its kind of an upgrade since Developer Edition is equivalent to enterprise in-terms of features.

A straight Start-DbaMigration (from dbatools) will copy all stuff.

I would suggest to provision new machines and just use dbatools to migrate off and then decomm the old standard edition.

Just be aware, if in PROD you are using Standard Edition and your devs started using Enterprise features since they are available in Dev edition, then you will be in trouble as those wont work in PROD being in standard edition.