When Barack Obama was getting ready to leave office, he changed the succession with executive order 13762. That EO replaced 13557 which had replaced 13481. That EO replaced "the President's Memorandum for the Attorney General of December 8, 2006 (Designation of Officers of the Department of Justice)." It doesn't explain why it switched from a presidential memorandum to an executive order to make this change.
Are such routine adjustments to the DOJ succession list normal, or should they prompt scrutiny of the changes?
Yes, both Obama and George W. Bush updated the succession list. So this should be considered a normal activity.
Note that normal activities may still be reasonably scrutinized for their effect. But this is not, in and of itself, an activity that could only be done for nefarious purposes.
The basic issue here is that there is nothing saying that the most experienced attorney general needs to be in any particular location. The fourth slot (first among regular attorneys general) is generally someone in the District of Columbia area, but Obama changed it from Maryland to Virginia and then to DC. While they don't say that, this was most likely driven by personnel changes. I.e. the Maryland AG left, so Obama switched to the more experienced Virginia AG rather than the brand new Maryland AG.
Trump fired 46 of the Obama US Attorneys (as Bill Clinton had previously fired 93 in 1993). So it is unsurprising that he wanted to change the succession. He could have put the people that he wanted in the previously selected roles, but he apparently preferred to make the assignments differently. Note that Jeff Sessions was probably the one actually making those choices with Trump's concurrence.
Do we have any information on why the White House favored the switch from Northern District of Illinois to Eastern District of North Carolina despite having set the previous list less than two months prior?
Obama had set the Attorney General of the Northern District of Illinois as the fifth in line of succession with EO 13762. For whatever reason, Trump had not been ready to change that in February but was in March. This may mean that in February, they had not yet chosen a location for the person they chose to take over the Eastern District of North Carolina. Or it may be as simple as that he was firing Zachary Fardon of Illinois in March but was leaving the NC US Attorney in place.
At the time that the executive order was made in March, the US Attorney for the Eastern District of Virginia was serving as the Deputy Attorney General. At the time the February EO was made, it effectively made him the acting Deputy Attorney General, replacing Sally Yates. So at the time, the succession mattered immediately, as there were vacancies in the first three spots. Sessions was not Attorney General and Yates had just been fired. So they needed to specify someone to be in charge of the department.
It's quite possible that when Trump gets his choices confirmed, that he will make changes again.
Would it be accurate to say that this change in succession would only have a real impact on DOJ business if Trump fired both Rosenstein and Brand?
Now, yes. But note that when he made them, neither Rosenstein nor Brand had been confirmed. It is perhaps more noteworthy that he hasn't bothered to change them since that time. He may be waiting until all the vacancies have been filled to do the next round of shuffling.
It appears that by replacing the Northern District of Illinois with the Eastern District of North Carolina, the EO replaces John R. Lausch Jr. as second in line outside of the DOJ org chart with Robert Higdon Jr.
Except that neither of them were appointed until long after March. I.e. at the time it replaced Zachary Fardon, not John R. Lausch, Jr.