Well, I do know that the new setup will detect whats going on and how. But its unclear as to if you are going to add the new hardware to the existing cluster or swap out the hardware and push the data to the newer servers. Your cluster setup can support upto 8 machines unless you have enterprise edition and then you can support quite a few more. Also you are correct as well If you could have a test environment its always ideal but the reality is not many places have boxes just to do this type of testing, simply because if there are any differences then it wont apply to what you have in production EXCEPT that you have done something similar to this in test. There is A LOT to clusters and a minor mistakes can cause major problems. Here is a pretty good article that may help.. Good Luck 🙂 And make sure you have a back out plan...this is a must.

http://www.sql-server-performance.com/articles/clustering/clustering_best_practices_p1.aspx