Seems too good to be true but I can't think of any reason why it wouldn't work. Let us know if you try it.
The only disadvantage would be waiting until fermentation is over to brew again, but if it makes upgrading to 10 gallon batches easier and cheaper for me, the advantages would outweigh by far.
Edit: I don't plan on getting a heating/cooling jacket, so that issue wouldn't be an issue in my case.