Bypassing version store for changing column datatype
Prefacing this by saying I think my idea is lame, but it is an idea. :)
You can try creating a new empty NVARCHAR column (of the appropriate size) and do batches of updates from OldVarCharColumn to NewNVarCharColumn at the rate your system can tolerate. You can even automate the entire update by iteratively looping and using a rolling range of the key (assuming it's numerical) for which rows, by setting a variable on each iteration (e.g. WHERE KeyField &gt;= @CurrentIterationValue - 1000 AND KeyField &lt; @CurrentIterationValue) and by using the WAITFOR DELAY command to pause between each iteration for any amount of time you specify (e.g. WAITFOR DELAY '00:05:00.000' will pause execution for 5 minutes).
Once your new column is fully updated you can then DROP the OldVarCharColumn.