Reset all IDs in a Large Table
One option is to create a staging table with a new ID as Row_number(). Consider the below sql:

begin transaction createstaging

select row_number() over (order by existingID) newIDValue, DataColA, DataColB, ...
into staging_table
from source_table 

truncate table source_table

commit transaction createstaging
GO

set identity_insert source_table on 
GO

insert source_table
(existingID, DataColA, DataColB, ...)
select 
newIDValue, DataColA, DataColB, ...
from staging_table 
order by newIDValue

GO

set identity_insert source_table off
GO

drop table staging_Table
GO


The truncate table in combination with the identity_insert should mean that you do not have to manually reset the identity seed on the source table.