$domain = 'someADDomain.local'
$userName = 'SomeUser'

$oldPassword = Read-Host -AsSecureString -Prompt "Enter the account's old password"
$newPassword = Read-Host -AsSecureString -Prompt "Enter a new password"

Set-ADAccountPassword -Server $domain -Identity $userName -OldPassword $oldPassword -NewPassword $newPassword


$Password = ConvertTo-SecureString "PlainTextPassword" -AsPlainText -Force $oldPassword
$Credential = New-Object System.Management.Automation.PSCredential ("$domain\$userName", $Password)
Set-ADAccountPassword -Credential $Credential -Server $domain -Identity $userName -OldPassword $oldPassword -NewPassword $newPassword
#                     -----------------------