You can activate/change the position of an end crystal beam with the command:

/data merge entity @e[type=minecraft:end_crystal,limit=1,sort=nearest] {BeamTarget:{X:1, Y:2, Z:3}}

But the command that should remove the beam is not working:

/data remove entity @e[type=minecraft:end_crystal,limit=1,sort=nearest] BeamTarget

video: 2020-03-1019-44-36.mp4 (ignore the bugged end crystal hitbox)