>
> Execute Statement 'Old.' || FieldName INTO :OldString;
>
> I get an error of Unknown Token 'Old'.
>


> Execute Statement 'Old.' || FieldName INTO :OldString;
> Execute Statement 'New.' || FieldName INTO :NewString;

Hi, i can't say I've used execute statement so i had to look it up, but it seems to me you are missing select and from.

Something like this:
execute statement 'select old.' || fieldname || ' from table_name where ...'

Sasha