Switching licences - what happens to mailboxes?

Question

Hi - I'm in the process of migrating multiple small business clients to Exchange Online.
Each migrated user will actually start with a trial version of E3, but they actually only need the Exchange Online Plan 1.
So, the process I'll follow is this:
set up users (with trial E3 licences)
migrate user accounts to Exchange Online
check that the migrations have been successful
purchase Exchange Online Plan 1 licences
apply Exchange Online Plan 1 licences to users
remove trial E3 licences from users
Basically, steps 5/6 involve switching licences - but when this happens, do mailboxes and all items remain intact? Or do they get wiped, meaning I need to migrate again?
Does that make sense?

Answer 1

I have just done something similar and ended up doing this in powershell.  Basically, got a list of users to migrate over in a text file, listed the AccountSku serviceplan and then removed all the Enterprise features I didnt need and then removed the
old license and applied the new.
Let me know if you still need a hand with this.
# This list imports users and licenses them to Office 365 / Exchange Online
<#
# Connect to MSOLINE
Import-Module msonline
$cred = Get-Credential
 Connect-MsolService -cred $cred
Get-Command –Module msonline
#>
# Get user list
$userlist = gc "C:\DataTemp\Phase4All.txt"
# SERVICE PLANS
# Get List of current Service Plans - and then list the ones you dont need as non licensed.
Get-MsolAccountSku | Format-Table AccountSkuId, SkuPartNumber 
Get-MsolAccountSku | Where-Object {$_.SkuPartNumber -eq 'ENTERPRISEPACK'} |
   ForEach-Object {$_.ServiceStatus}
# NON LICENSED PLANS - 
 # Disabled Plans
$disabledPlans= @()
$disabledPlans +="MCOSTANDARD"
$disabledPlans +="YAMMER_ENTERPRISE"
# $disabledPlans +="OFFICESUBSCRIPTION"
# Office 365 License - without LYNC Online
# $licoptions = New-MsolLicenseOptions -AccountSkuId Domain365:ENTERPRISEPACK -DisabledPlans $disabledPlans
$licoptions = New-MsolLicenseOptions -AccountSkuId "Domain365:ENTERPRISEPACK" -DisabledPlans $disabledPlans
### TRY Ctch
"Users not found" > "C:\DataTemp\ErrorList.txt"
foreach ($user in $userlist)
try
Get-MsolUser -UserPrincipalName $user
Set-MsolUser -UserPrincipalName $user -UsageLocation GB
Set-MsolUserLicense -UserPrincipalName $user -RemoveLicenses "Domain365:ENTERPRISEPACK" 
Set-MsolUserLicense -UserPrincipalName $user -AddLicenses "Domain365:ENTERPRISEPACK"  -LicenseOptions $licoptions
} catch {
# "User not found -
"Error with $user" >> "C:\DataTemp\ErrorList.txt"
Write-Host -fore Black -back Yellow "Imported list contains" $userlist.count "users"