On live servers you can't transmog polearms over swords, axes or maces (and vice versa), but on the Alpha it's a totally different story, as you can see in the header image above. No idea if this is an intended change or not (I hope it is) but it's certainly a welcome one for those of us who happen to carry a Hellreaver in our bags.
I tested it with axes, maces and swords - you can transmog a polearm to any of these, and any of these can be transmogged to a polearm as well. It also works with staves, as I transmogged my Zhar'doom over my Greatsword of Pride's Fall, and transmogged Zhar'doom itself to Gartok, Strength of the Faithful.
This is a pretty big unannounced change and one that I heartily approve of. Legendaries still can't be mogged to or from (I checked, believe me) but this is still a lot more flexibility. Druids will be able to go with a mace they like over a polearm or staff, or the reverse, and warriors, paladins and DK's will be able to go hog wild.