Sitefinity can be set to use either claims or forms based authentication, but not both at the same time. The authentication mechanisms are quite different and in general they can't be set up to work in parallel.
There are options in your case, though - you can start with forms based authentication and switch to claims at any point in the future after your migration is done.
Bear in mind that Sitefinity can
work with multiple user membership providers, i.e. you can connect it to multiple user data repositories at the same time. This is not affected by the authentication type you choose.
Feel free to get in touch with your go-to person from the Sitefinity team and discuss the options in further detail. It would help if you provided more details for the scenario you're envisioning using Sitefinity in.
Grisha 'Greg' Karanikolov
the Telerik team
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