If I run Perform on existing DB, profiler showing some odd queries, like this

ALTER TABLE [dbo].[Entity] DROP CONSTRAINT [FK_Entity_Field_Entity2];
...

then

EXEC sp_rename '[DB].[dbo].[Entity].[Field_Id]', 'TempField_Id', 'COLUMN';
ALTER TABLE [dbo].[Entity] ADD [Field_Id] uniqueidentifier;
UPDATE [dbo].[Entity] SET [Field_Id] = CAST([Entity].[TempField_Id]  AS uniqueidentifier);
CREATE INDEX [Entity1_FK_Entity2] ON [dbo].[Entity] ([Field_Id] ASC);

and finally

ALTER TABLE [dbo].[Entity] DROP CONSTRAINT [DF__Entity__FileVers__023D5A04];
ALTER TABLE [dbo].[Entity] DROP COLUMN [Field_Id];

Similar queries execute for many (all) linked fields and fields included in custom indexes

It seems that DO drop/create all indexed fields and rebuild all indexes

Why?