Those five installs on the one domain are covered by the old license.
When that license's upgrade period elapses, you get a 75% discount off of the upgrade period renewal price at that time.
You do not need to purchase additional licenses when your existing license's upgrade period elapses.
Now, on the other hand, if you needed to put another Threads up on another domain (or under another circumstance that would not fall under the old license), you would then need to purcahse a new license, which would cover only that one installation.