We still don't know when the T-Mobile/Sprint merger will close

T-Mobile could close the merger early in 49 states
It's pretty obvious that Magenta wants to get to work as soon as possible on combining its existing 5G network with Sprint's wealth of mid-band spectrum, but until the California Public Utilities Commission offers its blessing, that can't be done... in the Sunshine State.

Theoretically, that means separate T-Mobile and Sprint presences could be maintained in California in anticipation of the CPUC's formal approval, while "New T-Mobile" might in fact become a thing on April 1 everywhere else. Then again, that seems like a pretty major legal hassle that hasn't been seriously considered yet, at least according to unnamed "insiders" cited by PCMag.
Launching the united new carrier in 49 states on April 1 may not be worth the effort primarily because a final CPUC vote is expected to take place just 15 days later, but in the unlikely case said vote doesn't go in the merger's favor, Sprint could essentially become a regional wireless network operator for an indefinite period of time.
A D.C. District Court judge is also keeping T-Mobile waiting
The other small issue threatening to delay the completion of a merger in the works since April 2018 is something that was considered a formality from the get-go. While it's still unlikely that Judge Timothy Kelly will overrule the Department of Justice under his Tunney Act review of the conditions agreed for obtaining the DoJ green-light, it has already taken an unusually long time to reach a decision in this case.
Unlike the aforementioned CPUC examination of the deal, there's no official deadline for a judge ruling in D.C., which could derail the merger indefinitely. Of course, a decision has to be made sooner or later, and analysts fully expect this to be favorable to the birth of a strong new rival for Verizon and AT&T.

Meanwhile, it shouldn't come as a surprise to hear T-Mobile is already "doing the prep work" for combining its network with Sprint's wireless infrastructure in an attempt to lead the upcoming 5G revolution. This will naturally take time, but the bulk of Sprint subscribers could be transferred to T-Mobile's LTE network shortly after the merger is completed.
Before transferring everyone, however, T-Mo will need to entice Sprint subscribers using older phones to upgrade to new ones fully supporting Magenta's 4G speeds. Most importantly, T-Mobile plans to switch Sprint's Band 41 spectrum from 4G LTE to 5G, helping improve speeds to go nicely with the nationwide coverage of the "Un-carrier's" next-gen network.
By the way, in case you're wondering, the aim is for "all current T-Mobile 5G phones" to work properly on Sprint's 5G frequencies. Unfortunately, that won't work both ways, as "earlier" Sprint 5G handsets like the LG V50 ThinQ and OnePlus 7 Pro are not equipped with the necessary technology to support T-Mobile's low and high-band 5G frequencies.
Meanwhile, Samsung's "regular" Galaxy S20 variant is compatible with Sprint's mid-band and T-Mobile's low-band spectrum, lacking high-band functionality. Last but certainly not least, the S20+ and S20 Ultra are future-proofed to work on the "full merged network."