Change entire column value to same value?

mbr0wn Try using PnP PowerShell for bulk update of SharePoint list items.

Example: 

#Parameters
$SiteURL = "https://crescent.sharepoint.com/sites/PMO"
$ListName = "Projects"
 
#Connect to SharePoint Online site
Connect-PnPOnline $SiteURL -Interactive
 
#Get all Items from the list
$ListItems = Get-PnPListItem -List $ListName -PageSize 2000
 
#Update all list Items
ForEach($Item in $ListItems)
{
    #Update List Item Title as its ID value
    Set-PnPListItem -List $ListName -Identity $Item.Id -Values @{"Title"= $Item.Id} | Out-Null
    Write-host "Updated Item:"$Item.ID   
}

Source: SharePoint Online: Bulk Update All Items in Large List using PowerShell 

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