Minecraft – Can people change their name to the previous name

There is no way of changing that. Users pick what they want that to say.

In the "add server" menu, users can put a name in. (I usually just put a space, so that all i see it the MOTD)

The MOTD is all that you can change yourself for all people.

There is no command to change a player's name.

As a workaround, you could fake it by hiding the player's name whilst teleporting a named entity onto them.

To hide a player's name tag, put them on a separate team and change the nametagVisibility option:

/team add HerobrineTeam
/team modify HerobrineTeam nametagVisibility never

Then summon a named entity, such as a marker armorstand:

/summon armor_stand ~ ~ ~ {CustomName:'"Herobrine"',CustomNameVisible:1b,Marker:1b,Invisible:1b}

You can teleport that entity to the player on a clock with something like:

/execute at @p[team=HerobrineTeam] run tp @e[type=armor_stand,name=Herobrine] ~ ~1.7 ~