Avoiding failed upgrade due to "Incompatible editions"

Problem

You run the migration as described in Upgrading 2.x to 3.y where the source is Nexus 2 OSS (version 2.14.2-01 in this example), and the destination is Nexus 3 Professional (3.2.0-01), but you get an error when starting the Upgrade:

Incompatible editions: nexus/OSS/2.14.2-01 to nexus/PRO/3.2.0-01

Here is a log snippet from Nexus 3 for the above error:

2017-01-03 16:23:57,938-0500 ERROR [qtp2013436960-145] admin org.sonatype.nexus.extdirect.internal.ExtDirectServlet - Failed to invoke action method: migration_Assistant.connect, java-method: com.sonatype.nexus.migration.ui.AssistantComponent.connect
java.lang.IllegalStateException: Incompatible editions: nexus/OSS/2.14.2-01 to nexus/PRO/3.2.0-01
	at com.google.common.base.Preconditions.checkState(Preconditions.java:518) [com.google.guava:20.0.0]
	at com.google.common.base.Preconditions$checkState$0.callStatic(Unknown Source) [na:na]

Solution 

First upgrade Nexus 2 OSS to Nexus 2 Pro, which you can download from here: Download Nexus Repository Manager 2

Your Nexus 3 license can be used in Nexus 2 PRO to perform the upgrade.