From a recent IPNN:

Important Freeze Information

If you are bidding the B747-400, be aware that you will incur a two-year freeze if you upgrade or transition to this new aircraft type, unless you are displaced/realigned to this aircraft type. This is per Article 14.N.2.f.

The two year freeze will begin on MARCH 11, 2008, which is the award date of Realignment Vacancy Bid 08-101. However, if you displace or realign to the 747-400 you do not incur that freeze. Also note that if you are in this special new aircraft type freeze, it does not prevent you from a future upgrade during the freeze period.

IF YOU ARE IN A FREEZE, IT DOES NOT FREEZE YOU FROM GETTING AN UPGRADE BID:

IPA Scheduling's RK says it is important to note that the various freezes in the Agreement (such as 14.E.2., 14.N.2.f.) only block you from getting a TRANSITION (Captain to Captain or FO to FO) vacancy award but never an UPGRADE (SO to FO or FO to Captain) vacancy award.