You can turn automatic identity off when you need to using
SET IDENTITY_INSERT sometableWithIdentity ON
SET IDENTITY_INSERT sometableWithIdentity OFF
And change the seed values using DBCC CHECKIDENT (Transact-SQL) - SQL Server | Microsoft Learn
But ... If you need to do this more than once in the life of the DB, it's a sure sign that your DB design is wrong, and you really, really should be using GUIDs instead.
You are going to have to be really, really careful here: if multiple users try to do this at the same time, you are almost bound to get corrupted data at some point.
Would I do it? Nope. I'd redesign my DB to make my whole life a load easier later ...