As far as I can tell, doing that "reservation" says you agree to accept a forced 3GB download when Windows 10 is released. Microsoft will "push" it to your computer. Then you're expected to install it, overwriting your existing Windows 7. First, of course, backup your system. Backup, backup, backup and check to make sure the backup is good.
HOWEVER... if I were you I would wait for a couple months before updating, give them a chance to find and fix the worst of the bugs. It'll be free for a year after release, so there's actually no rush, even though they will pressure you.
> I miss my XP-PRO. I find it easier than this Windows 7.
If that's the case, then I would definitely say to wait a while. In the meantime, you might take a look at a computer that has Windows 8 on it and try to use it. See what you think. You may be surprised, either pleasantly or not, but Windows 10 is a lot more like Windows 8, than it is like Windows 7. And it's nothing like XP.
I expect there will be numerous add-on products that try to make Windows 10 more like Win 7 or Win XP, because a ton of people still crave the simplicity of the old XP interface.
You know I am getting a bit old (70) to learn a new system. The technology is for the young. I have been away from it for so long that all I do is fix problems and that is a challenge now. Thank you for your input, I really do value it. I did remember that you all never updated until after several months allowing for time to get any big bugs out.