<p>I am writing a DB upgrade script that will check to see if an index has the right two columns defined.  If it doesn't, or if it only has one of them, then I will DROP it (is there a way to ALTER an index?) and then recreate it with both.</p>

I don't have a database immediately on-hand to test this, but you should be able to see if a column exists in an index by using the following IF EXISTS statement.
I'm not sure whether you can alter an index on the fly.
```
```
IF EXISTS
(
   SELECT MyIndex.Name AS IndexName, 
          Columns.name AS ColumnName 
   FROM sys.indexes MyIndex
   INNER JOIN sys.index_columns IndexColumns 
      ON  MyIndex.index_id = IndexColumns.index_id
      AND MyIndex.object_id = IndexColumns.object_id 
   INNER JOIN sys.columns Columns
      ON  Columns.column_id = IndexColumns.column_id 
      AND IndexColumns.object_id = Columns.object_id 
   WHERE Columns.name = 'ColumnName'
   AND MyIndex.Name='IX_MyIndexName'
)
```
```