--get the type
declare @type nvarchar (50) =
(
    SELECT
        TYPE_NAME(system_type_id)
    FROM
        sys.columns
    WHERE
        name            = 'measurement'
        AND [object_id] = OBJECT_ID('[dbo].[tevent]')
)
if @type<>'decimal'
begin
    select
        @type
    IF NOT EXISTS
    (
        SELECT *
        FROM
            [information_schema].[columns]
        WHERE
            table_name       = 'tevent'
            AND table_schema = 'dbo'
            AND column_name  = 'measurement_helper'
    )
    BEGIN
        --add tmp
        ALTER TABLE tevent add measurement_helper decimal(15,2) NOT NULL DEFAULT '0.00'
        --Copy from old nvarchar column to new column with trim and conversion.
        update
            tevent
        set measurement_helper=CONVERT(decimal(15,2), REPLACE(LTRIM(RTRIM(measurement)), ',', '.'))
        where
            ISNUMERIC(measurement)=1
        -- Drop old nvarchar column.
        IF EXISTS
        (
            SELECT *
            FROM
                dbo.sysobjects
            WHERE
                id       = OBJECT_ID(N'[dbo].[DF_tevent_measurement]')
                AND type = 'D'
        )
        BEGIN
            ALTER TABLE [dbo].[tevent]
            drop CONSTRAINT [DF_tevent_measurement]
            ALTER TABLE tevent
            drop column measurement
        END
        else
        begin
            ALTER TABLE tevent
            drop column measurement
        end
        -- Rename numeric column to old column name.
        IF EXISTS
        (
            SELECT *
            FROM
                [information_schema].[columns]
            WHERE
                table_name       = 'tevent'
                AND table_schema = 'dbo'
                AND column_name  = 'measurement_helper'
        )
        begin
            EXEC sp_rename
                'tevent.measurement_helper',
                'measurement',
                'COLUMN'
        end
        -- add dropped constraint
        IF NOT EXISTS
        (
            SELECT *
            FROM
                dbo.sysobjects
            WHERE
                id       = OBJECT_ID(N'[dbo].[DF_tevent_measurement]')
                AND type = 'D'
        )
        BEGIN
            ALTER TABLE [dbo].[tevent] ADD CONSTRAINT [DF_tevent_measurement] DEFAULT ('0') FOR [measurement]
        END
    end
    else
    begin
        select
            'helper column exists'
    end
end
else
begin
    select
        'already modified'
end