If you have enabled SFB Hybrid with shared sip address space to SFB Online, then a proper migration of moving the users from on prem sfb to TeamsOnly is required.
If you have users hosted in Skype for Business with full telephony features, auto attendants and call queues that will be a major impact.
And users will lose their meetings hosted in Skype , their Skype Contacts and favourites as well.