Have I lost something? Do you need to change datatype of the table column or you need to change the datatype and values as well?

If only column change, than the maximum available speed can be achieved by simple altering the column:

alter table [yourtable] alter column [yourcolumnname] bigint

If you have indexes/relationships on this column, they must be dropped and recreated in at additional cost.

If you are talking about value change as well, then I guess the any type of the bulk load is about the fastest options you have: BCP, BULK INSERT, SELECT INTO.