Users will see the increased data beginning with their next plan renewal after the 28th.
T-Mobile hasn't announced the change yet, but the TmoNews story includes a screenshot detailing the data increase. The screen grab appears to be from an internal site for T-Mobile customer support personnel.
In addition to data, the $50 plan includes unlimited voice minutes, unlimited messaging and off network voice and SMS roaming. The 500 MB of data is not throttled and users can expect near LTE speeds in areas where T-Mobile has implemented 42 Mbps HSPA+.
This is a nice move by T-Mobile and makes the $50 plan much more competitive.
Update: It's the evening of Jan 28 and T-Mobile's Monthly 4G Plans page is still showing The $50 plan as coming with only 100 MB of 4G data. Either the plan was delayed, T-mobile management has changed its mind or the rumor was a hoax. Time will tell.
Source and Image: TmoNews