How to change the datatype of column without deleting it indexes;
The data type of columns included in an index cannot be changed unless
the column is a varchar, nvarchar, or varbinary data type, and the new
size is equal to or larger than the old size.

With this info from the knowledge base a suggestion could be to alter column to a nvarchar(4000) or something like that.
Remember that you can't index a varchar(max) or nvarchar(max) field.