Hi vmxmr,
From what I understand you can downgrade a license to an earlier version as it certainly has been mentioned here before at the forum.
After you bought a license, go to: Login - My VMware
Select "License Keys" menu option on the left. Your license should show up.
You then get an option at the top "I want to" where you can select "Downgrade license keys" from a combo box.
That should be the option you are looking for.
But I admit I have not done that myself and as I'm not working for VMware I'm not guaranteeing it to work either.
If you are unsure about the procedure I'm afraid you will have to contact VMware sales, they are the only ones who can help you with this.
Your question on "temporary usage" appears to suggest you want to use the same license for 2 installs -one Fusion 7 and one Fusion 5- at the same time, I'm unsure and doubtful that that is supposed to work.
The forum isn't frequented by VMware Sales people afaik, or to put in another way:
For your information, the Community Forum is not an official support-forum and is not actively monitored by VMware Support or Product Managers. This is a community based forum where VMware Fusion users help other users.
You will have to contact VMware Sales for a more definite answer on this.
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Wil