Characters Traveling/Active between Regions

[Steve]

How easy is it, from both Lore and a practical standpoint, is it now, to go from the Underdark, to the Upperdark, and further on to the Surface, or vice versa?

Is this "journey" to be understood and developed in RP as easy as it seems to be applied by Players, as of late? Have we taken a turn in the Sword Coast contemporary history that, Drow are common on the Surface, and Surfacers, just as common in the Basement?!!?

Can we get some guidelines about such behavior—a Character being active between the Underdark/Upperdark/Surface-Soubar—such that it has some practical relevance? And moreso, some reaction from the NPC forces of the greater Sword Coast that might react to such newly established occurrences?

Or, sarcastic as it sounds, is travel/activities between regions of great distance and travail, as easy as it is to encounter and slay dragons (regarding mobs)?

[Steve]

Would like to know if DMs have a response to this query. Cheers.