Hello @KMA10536 ,
Yes, you can do this using the "Remove from list" function in the actions menu. You'll first head to your "original list," and then select-all using the topmost checkmark box. You'll then click Actions at the top of the page > Remove from list > select the main list, and that'll get those 400 contacts in the "original list" removed from the "main list." For step-by-step and visual guidance on contact removal, you can reference this article.
Keep in mind however, that if your "original list" contains more than just those 400 contacts, and you don't want those others removed from the "main list," then you may be forced to use a more manual removal process — AKA individually selecting and removing the unwanted contacts from your "main list."
If particularly dire, and if the list addition has occurred within the last 24 hours, you can call our general support to submit a contact rewind request with our higher level technical team. It's advised that if you're wanting them to do this, you submit this request as soon as possible before closing time today (since tomorrow is a holiday). Additionally, I would recommend not making any significant changes to your contacts and lists (including list additions and renames), nor send / have any emails scheduled to the affected lists, until the higher level technical team has completed the task and notified you. Please also keep in mind this process is not a guarantee, they will simply do their best to dive into our system's logs and see if they can restore the lists and contacts' original memberships.
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William A
Community & Social Media Support