Allowed User License Changes: Upgrades, Downgrades, Switches?

An infrequent but significant question that companies ask about Salesforce user licenses is: Can I switch from this license to that license? At the time of Winter '14, I could not find an official matrix of allowed user license changes. However, the question lingers, and I balked at the thought of manually trying to change user licenses to see what happens every time I needed an answer.

Using what I hope is a valid approach, I've compiled a matrix of allowed changes, using what licenses existed in my Winter '14 test org. I'm sharing the high-level summary of my approach in case anyone case to reproduce the same results.

1. Create test profiles for every user license
2. Run an Apex test to compile the results of every possible change among profiles

Below are the user licenses that were analyzed and included in these results:

• Chatter External
• Chatter Free
• Chatter Only
• Content Only
• Customer Community
• Customer Community Login
• Customer Portal Manager Custom
• Customer Portal Manager Standard
• Force.com - One App
• Gold Partner
• High Volume Customer Portal
• Knowledge Only
• Partner Community
• Partner Community Login
• Salesforce
• Salesforce Platform
• Siteforce Only
• Work.com Only

My wish is that anyone asking the same question can now refer back to this matrix for a quick answer. All feedback on the accuracy of the results will be greatly appreciated.