Update AzureAD User otherMails attribute to allow conversion from AAD Member to AAD B2B Member. Associated blogpost https://blog.darrenjrobinson.com/convert-azure-ad-users-from-members-to-b2b-members/

Update otherMails AAD User attribute.ps1

Import-Module MSAL.PS -RequiredVersion 4.7.1.1

$tenantID = "myTenant.onmicrosoft.com"
$clientID = "yourRegistedAppID"
$clientSecret = (ConvertTo-SecureString "yourRegistedAppSecret" -AsPlainText -Force)

$accessToken = Get-MsalToken -clientID $clientID -clientSecret $clientSecret -tenantID $tenantID -ForceRefresh | Select-Object -Property AccessToken

$users = Invoke-RestMethod -Headers @{Authorization = "Bearer $($accessToken.AccessToken)" } `
    -Uri  'https://graph.microsoft.com/v1.0/users' `
    -Method Get

$convertUser = $users.value | Select-Object | Where-Object { $_.displayName -eq "Partner User" }
$externalEmail = @{"otherMails" = @("foreign.address@partner.com.au") }

try {
    Invoke-RestMethod -Headers @{Authorization = "Bearer $($accessToken.AccessToken)"; "content-type" = "application/json" } `
        -Uri  "https://graph.microsoft.com/v1.0/users/$($convertUser.id)" `
        -Method Patch `
        -body ($externalEmail | ConvertTo-Json)
    
    # check user
    $updatedUser = Invoke-RestMethod -Headers @{Authorization = "Bearer $($accessToken.AccessToken)"; "content-type" = "application/json" } `
        -Uri  "https://graph.microsoft.com/v1.0/users/$($convertUser.id)?`$select=id,displayName,userPrincipalName,otherMails" `
        -Method Get 
}
catch {
    Write-Error $_
}