I would like to use BitLocker to encrypt my hard drive, but I have the standard 8.1 version, which will become Windows 10 Home version when I upgrade. Neither of those editions come with BitLocker. Do I have to upgrade to the Pro version to get it? If yes, what would be the best thing to do? Upgrade to 8.1 Pro, then upgrade to 10 Pro? Or upgrade to 10 Pro after installing 10 Home?

Isn’t there any way to use BitLocker on a standard Windows version?

At the time of writing, Windows 10 will be rolled out tomorrow! It will be free to download for one year, so I don’t need to hurry… right?

Yes, only Pro and Enterprise editions have BitLocker

If yes, what would be the best thing to do? Upgrade to 8.1 Pro, then upgrade to 10 Pro? Or upgrade to 10 Pro after installing 10 Home?

I would take the free offer and upgrade Windows 8.1 to Windows 10 Home then purchase a Windows 10 Pro Pack for $99 and perform an Easy Upgrade to Windows 10 Pro

How to Perform an Easy Upgrade in Windows 10

Isn’t there any way to use BitLocker on a standard Windows version?

You can use third party alternatives such as Truecrypt but you will have to determine if they are compatible with Windows 10.

At the time of writing, Windows 10 will be rolled out tomorrow! It will be free to download for one year, so I don’t need to hurry… right?

That is correct, you have until July 28th 2016 to take advantage of the free upgrade.