Can I install the Windows 10 test version, or whatever it is called, now, and just update later to the final build whenever it becomes available to the public?
I've just re-installed some days ago, but I'm an insider. (Called Inisider Preview :D)
I had to turn on to get Insider builds, so theoritically if I hadn't changed this setting I would have a full os.
Yes lots of people think the 10240 build is very close (or it is) the final build. (I read about (jokes) 10240/1024=10 and so on)
I heard about, that who is using windows 10 Insider Preview now and then wants a full stable build when it will come out they can get, but if the user wants he/she can stay at insider build, and get "beta" updates for their system. But now you cant get the insider preview, because the 10240 only avalible trought windows update.
In conclusion, you probably should can use the 10 as a fully functional os, and very likely the 10240 is final. Probably you will be able to update.
Update: I read just now that the 10240 build IS the final build. I read it in Hungarian i try to translate it to english: "This will be the version which will be installed on new computers, and after 29th of July this will be the version, that everyone can update to" Sources: www.prohardver.hu, www.hsws.hu, www.computerworld.com, lots more
Technically you should be able to update/upgrade to the latest retail version when it comes out. However from past history this may not be the best way to go about it.
I have seen so many issues going from Microsoft "beta" products to full retail that its almost certain there is a risk of issues occurring later on.
Its always better to install a fresh retail version of windows when you have a BETA or other pre-retail version. But yea technically you should be able to.