Alter table Alter column Performance
Referencing Aaron Bertrand's answer to a similar question, he says:


  Depending on what change you are making, sometimes it can be easier to
  take a maintenance window. During that window (where nobody should be
  able to change the data in the table) you can:
  
  1). drop any indexes/constraints pointing to the old column, and
  disable triggers
  
  2). add a new nullable column with the new data type (even if it is
  meant to be NOT NULL)
  
  3). update the new column setting it equal to the old column's value
  (and you can do this in chunks of individual transactions (say,
  affecting 10000 rows at a time using UPDATE TOP (10000) ... SET newcol
  = oldcol WHERE newcol IS NULL) and with CHECKPOINT to avoid overrunning your log)
  
  4). once the updates are all done, drop the old column
  
  5). rename the new column (and add a NOT NULL constraint if
  appropriate)
  
  6). rebuild indexes and update statistics
  
  The key here is that it allows you to perform the update incrementally
  in step 3, which you can't do in a single ALTER TABLE command.
  
  This assumes the column is not playing a major role in data integrity
  - if it is involved in a bunch of foreign key relationships, there are more steps.