Tp From Relative Position to Another Relative Position

1.12

This is actually a surprisingly hard thing to do with 1.12 commands. The only way I found is with a marker entity, the command inside the command block is then:

summon armor_stand ~ ~ ~ {Tags:["teleportMarker"],NoAI:1,NoGravity:1,Invisible:1,Marker:1}

Execute this command inside the command block only once per block. How you do that depends on your system.

Then tag everybody in your first area:

/execute @e[tag=teleportMarker] ~123 ~456 ~789 scoreboard players tag @a[r=42] add toTeleport

Then teleport them to the second area:

/tp @a[tag=toTeleport] ~987 ~654 ~321

Then clean up:

/scoreboard players tag @a[tag=toTeleport] remove toTeleport

And when you're done teleporting people around, you can kill the marker amour stands again:

/kill @e[tag=teleportMarker]

Killing only one of the markers works best with whatever you use to place/activate the command blocks, since that already knows the coordinates of that command block and therefore also of the armour stand.

1.13

In 1.13 you don't need a marker entity, since there is now /execute positioned. So you can just put

execute positioned ~123 ~456 ~789 run tag @a[distance=..42] add toTeleport

into the command block instead of summoning an entity and then executing from it.