The datatype of column <Columnname> in the <Tablename> table can't be changed because it participates in index <Indexname>. (Visual Database Tools)
The sum of the lengths of all columns in an index can not exceed 900 bytes. Changing to the specified datatype would exceed the maximum index size of <3d> bytes.
For more information, see How to: Modify Column Data Types (Visual Database Tools) and How to: Create Indexes (Visual Database Tools).