You can update to Windows 10 Technical Preview now, if you join the Insider Program, but you might want to consider waiting for 6 weeks and just update to the final retail version, you will save yourself some hassles with the multiple updates that will appear between then and now.
There are still some bugs in the preview versions.
Personally at this late date I'd wait.
If you don't like the Windows 8.1 interface you could install and use Classic Shell for the next 6 weeks instead of trying to get used to Windows 8.
You could use the Reserve Your Copy app, now, you should see an icon show up in the lower right of the screen at some point if it's not already there.
I suppose the first thing we need to discuss is if you upgrade to 10 you may not be able to get back to 8.1. If you take precautions, you should be OK.
Second, you can upgrade now and have the Preview Build activated. There a many unknowns about how the process will work and folks at Microsoft seem to put out different procedures almost every day. The preview builds so far still have problems and the update process is not yet set in stone. You may want to wait a month and install the final version and you will have a year to get the upgrade without additional cost.