Set-MailUser Bulk Operations - Issues Updating LegacyExchangeDN

I'm performing a Cross-Tenant O365 migration using the Microsoft way.
Resources:
https://learn.microsoft.com/en-us/microsoft-365/enterprise/cross-tenant-mailbox-migration?view=o365-worldwide
https://blog.matrixpost.net/office-cross-tenant-mailbox-migration/

The issue I have is in the Identity mapping phase where you prep the Mail Users in the target tenant by setting their Primary SMTP, ExchangeGUID, and LegacyExchangeDN. When I perform this operation in bulk, the LegacyExchangeDN fails to change.

I've proven via Exchange Online that I can modify these values with the below cmds:

set-mailuser -identity "CT Migration" -ExchangeGuid 55555555-5555-5555-5555-555555555555
Set-MailUser -Identity “CT Migration” -EmailAddresses @{add=”X500:/o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=55555555555555555555555555555555555555555-0″}

The issue I face is specifically related to the LegacyExchangeDN value not getting modified (even though the other values are modified).

Below is the script I made - keep in mind I'm importing a csv with the values I want to set. Also, I'm exporting the get results prior AND after the set cmd:

# Step 1: Bulk Get-MailUser and export to CSV
$csvFile = "C:\Scripts\SetMailUserBulk.csv"
$outputFile = "C:\Scripts\PriorToSet.csv"

$users = Import-Csv $csvFile
$attributes = "Name", "LegacyExchangeDN", "ExchangeGUID", "UserPrincipalName", "PrimarySMTPAddress", "ExternalEmailAddress"

$users | ForEach-Object {
    $user = $_
    $mailUser = Get-MailUser -Identity $user.Identity | Select-Object $attributes
    $mailUser | Select-Object $attributes | Export-Csv -Append -Path $outputFile -NoTypeInformation
}

# Step 2: Bulk Set-MailUser
$updatedUsers = Import-Csv $csvFile

$updatedUsers | ForEach-Object {
    $user = $_
    $mailUser = Get-MailUser -Identity $user.Identity

    Set-MailUser -Identity $user.Identity `
        -ExchangeGuid $user.ExchangeGuid `
        -EmailAddresses @{Add = "X500:$($user.LegacyExchangeDN)"}
}

# Step 3: Bulk Get-MailUser after modification and export to CSV
$modifiedOutputFile = "C:\Scripts\AfterSet.csv"

$updatedUsers | ForEach-Object {
    $user = $_
    $mailUser = Get-MailUser -Identity $user.Identity
    $mailUser | Select-Object $attributes | Export-Csv -Append -Path $modifiedOutputFile -NoTypeInformation
}

The csv LegacyExchangeDN value in the csv I import has the following format:

"/o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=555555555555555555555555555555555555555555555-1c"

I've modified the LegacyExchangeDN values in the imported csv and added the @{Add = "X500: section of the set cmd. I get no errors on the output - the script runs fine - the only issue is that the LegacyExchnageDN value is not getting modified from the set-mailuser cmd.

So it turns out my output did not include the -EmailAddresses attribute and was not reflecting the changes the set cmd made.

I also concatenated the x500 proxy in the script and created two seperate set-mailuser cmds.
Below is the final script:

# Step 1: Bulk Get-MailUser and export to CSV
$csvFile = "C:\Scripts\SetMailUserBulk.csv"
$outputFile = "C:\Scripts\PriorToSet.csv"

$users = Import-Csv $csvFile
$attributes = "Name", "EmailAddresses", "LegacyExchangeDN", "ExchangeGUID", "UserPrincipalName", "PrimarySMTPAddress", "ExternalEmailAddress"

$users | ForEach-Object {
    $user = $_
    $mailUser = Get-MailUser -Identity $user.Identity | Select-Object $attributes
    $mailUser | Select-Object $attributes | Export-Csv -Append -Path $outputFile -NoTypeInformation
}

# Step 2: Bulk Set-MailUser
$updatedUsers = Import-Csv $csvFile

$updatedUsers | ForEach-Object {
    $user = $_
    $mailUser = Get-MailUser -Identity $user.Identity
    $Email = "X500:" + "$($user.LegacyExchangeDN)"
    write-host "$Email"

    Set-MailUser -Identity $user.Identity -ExchangeGuid $user.ExchangeGuid -PrimarySmtpAddress $user.PrimarySMTPAddress
    
    Set-MailUser -Identity $user.Identity ` -EmailAddresses @{Add = "$Email"}
}

# Step 3: Bulk Get-MailUser after modification and export to CSV
$modifiedOutputFile = "C:\Scripts\AfterSet.csv"

$updatedUsers | ForEach-Object {
    $user = $_
    $mailUser = Get-MailUser -Identity $user.Identity
    $mailUser | Select-Object $attributes | Export-Csv -Append -Path $modifiedOutputFile -NoTypeInformation
}