I don't know if i will prefer override the %JSONExportToString and %JSONExportToStream or  wrapping but that dirty trick..
 

Class test.elementList Extends (%ListOfObjects, %JSON.Adaptor)
{



/// The type (class name) of the elements stored in the collection.

Parameter ELEMENTTYPE = "test.element";



/// Returns this object as a JSON Stream
Method %JSONExportToStream(ByRef objStream As %Stream.Object, %mappingName As %String = "") As %Status
{

    #Dim objStream As %Stream.TmpCharacter = ##class(%Stream.TmpCharacter).%New()

    Do objStream.Write("[")

    For i=1:1:..Size
    {

        Do ..GetAt(i).%JSONExportToStream(.objStream, %mappingName)

        If (i<..Size)
        {

            Do objStream.Write(",")
        }
    }

    Do objStream.Write("]")

    Return $$$OK
}



/// Returns this object as a JSON string
Method %JSONExportToString(ByRef jsn As %String, %mappingName As %String = "") As %Status
{

    #Dim jsonElement As %String = ""

    Set jsn = "["

    For i=1:1:..Size
    {

        Set jsonElement = ""

        Do ..GetAt(i).%JSONExportToString(.jsonElement, %mappingName)

        Set jsn = jsn_jsonElement

        If (i<..Size)
        {
            Set jsn = jsn_","
        }
    }

    Set jsn = jsn_"]"

    Return $$$OK
}

}

Im executing inside a sql procedure the next sentence:

Set columnLength = ""
&SQL(SELECT character_maximum_length INTO :columnLength
     FROM information_schema.columns
     WHERE table_name = 'MYTABLE'
     AND   column_name = 'MYCOLUMN'
     AND   table_schema = 'MYSCHEMA')
If ((SQLCODE = 0) && (columnLength '= 32000))
{
    &SQL(ALTER TABLE MYTABLE ALTER COLUMN MYCOLUMN VARCHAR(32000))
    IF ((SQLCODE '= 0) && (SQLCODE '= 100))
    {
        SET %sqlcontext.%SQLCODE = SQLCODE
        SET %sqlcontext.%Message=%msg
        Quit
    }
}

And im getting the error:

%msg: <Insufficient Privilege For Operation at location: DDL ALTER TABLE Modify Column Datatype>]

And i try with NO Data and the same error. 

I added a %All rol to my user connected and then it's possible to alter the table. So i need one permission or priviledge that im missing....

the result is true for this command, so i have access but doesnt work.