Good news for all! Windows 10 will be free for all Windows Phone 8.1 users.
http://www.windowscentral.com/windows-10-b...-phone-81-users

The new OS will also be free for Windows 7 and Windows 8.1 users. We will also get new Office that is more powerful like the desktop Office. I can't wait for this.

This is from lowyat
http://www.lowyat.net/2015/01/windows-10-f...-and-8-1-users/

It will be free but you must upgrade within the first year the operating system launches. That means this year.

This post has been edited by zack9999: Jan 22 2015, 07:03 AM

already mentioned before, all lumia wp8.1 phones upgradable to Windows 10 mah...

you got one year to upgrade for free once Windows 10 released...let say Windows 10 release on Sept 2015, you got one year timeframe to upgrade until Sept 2016.

if you don't upgrade within one year timeframe and you want upgrade later, you need to pay la...

i think MS want all you guys upgrade asap so they can show all developers that they got 1 billion users and don't tok kok that MS don't have market share...

you see how MS develop the Universal app for Outlook, Photos, etc...and real live integration with each devices....very well integrated

Indeed it was mentioned before about the upgradable but no real comfirmation from the horse mouth. Still lacking info whether 512 MB memory can run Windows 10. But I like the direction that OS upgrade is free like Apple has done.
The universal app is good as right now one of the considerations for me to buy an app for Windows phone is whether I can install it on my desktop too.

well, turns out that windows 10 for phone will be made available for 512mb device
source

Heard that the preview will be released sometime this week.

yup, that's what they said on Wcentral.
source