I would say the process is described well in docs
https://docs.microsoft.com/en-us/microsoftteams/operator-connect-configure#move-numbers-from-calling...So I would consider unsinging the number first as suggested.
These days you have also different commands on newer powershell Teams module which lets you specify new phone number type on assigned number.
https://docs.microsoft.com/en-us/powershell/module/teams/set-csphonenumberassignment?view=teams-ps
