This is one of the most detailed answers I have seen yet for what is happening to White Dwarf and especially those of you that currently hold subscriptions. Here is an answer from GW Customer Service regarding the changes.
via Games Workshop Customer Service
Thanks for the email, there are indeed a few changes to the White Dwarf, from February the White Dwarf team will be releasing 2 magazines, White Dwarf weekly and Warhammer Visions.
White Dwarf weekly is exactly that; a 36 page weekly magazine, come February we will be releasing new products weekly rather than monthly, which means more great models and hobby supplies to enjoy. The White Dwarf Weekly will contain everything you need to know about all the week's new releases and latest Hobby news. You'll also find new features, new modelling and painting techniques, new rules, new columnists and much more.
Warhammer Visions is a completely new monthly magazine from the White Dwarf team. It will be over 230 pages full with a visual feast of Citadel Miniatures from the Warhammer, Warhammer 40,000 worlds and the lands of Middle Earth, with all your favourite sections from the old White Dwarf magazine from Army of the Month and Blanchitsu, to Kit Bash and Paint Splatter, along with new articles.
The White Dwarf weekly will be available to purchase every saturday from your local Games Workshop and independent retail store, as well as from the Games Workshop website.
As a subscriber, your subscription will now transfer over to the monthly Warhammer: Visions magazine, however you will also initially receive a free copy of the 1st issue of White Dwarf Weekly so you can see the difference between the new magazines. You should also receive a letter explaining the changes to the subscription, and if you would like to make any amendments to the subscription the letter should detail how to do so. You should receive this around the 1st February, (possibly a few days thereafter depending on the postal system in your area).
After the initial delivery you will receive the new Warhammer Visions issue each month, which will be released on the first Saturday each month, and as a subscriber you should receive the magazine a few days before the release in stores.
If you are a current subscriber the price for the subscription will not change until you need to resubscribe.
We hope you will enjoy the new magazines.