Unable to assign this license

I think your problem is with the other SfB related services:

[11:47:13][O365]# (Get-MsolAccountSku | ? {$_.AccountSkuId -eq "MOD789510:ENTERPRISEPREMIUM"}).ServiceStatus | ? {$_.ServicePlan.ServiceName -like "MCO*"}

ServicePlan ProvisioningStatus
----------- ------------------
MCOEV       Success
MCOMEETADV  Success
MCOSTANDARD Success

So if you only disable MCOSTANDARD, you get the error. To disable it properly, you need to disable them all:

[11:48:10][O365]# Set-MsolUserLicense -UserPrincipalName LidiaH@MOD789510.onmicrosoft.com -LicenseOptions $newplan
[11:48:14][O365]# (Get-MsolUser -UserPrincipalName LidiaH@MOD789510.onmicrosoft.com).licenses.servicestatus

ServicePlan                 ProvisioningStatus
-----------                 ------------------
Deskless                    Success

LOCKBOX_ENTERPRISE          Success
EXCHANGE_ANALYTICS          Success
MCOEV                       Disabled
MCOMEETADV                  Disabled
MCOSTANDARD                 Disabled
EXCHANGE_S_ENTERPRISE       Success
SHAREPOINTENTERPRISE        PendingInput
SHAREPOINTWAC               PendingInput