Island and Teams mode

Good Morning and a happy new year.

I am working with an organisation (A) that has its own O365 Tenancy

Installed O365 to 11,00 staff along with migrating data with SharePoint.

The legacy system is Skype business server on premise, used for voice along with 2 other close partner organisations(B and C)

Each Organisation has it own tenancy for O365 but share the Skype for Business Server.

Organisation A will have all its members by the end of January using Teams voice.

Organisation A has their tenancy set as island mode at the moment, because of Skype Voice.

Can they change this to Teams once they have come off Skype and will it affect the other 2 organisations.

Your experiences will be most valuable.