I am not sure that Microsoft will force updates to 10. (Updates not upgrade.)
But if you sign up to reserve? it, it will download the UPGRADE when it is your turn. The install will not take place until you activate it.
I have seen nothing about updates being automatically installed unless you have that feature turned on.
To the person who will wait until SP1 is out, may be waiting a long long time. MS states that there will only be updates, no SP1.