The original show was scheduled for Kiel Opera House (smaller venue) but due to the popularity of Poco (very big in St. Louis) the shows were moved to Kiel Auditorium (16,000).
If I recall, the headliner was switched back and forth (Yes at these shows). I don't recall if I attended on the 16th or 17th (17th I think) but the show was absolutely awesome - both groups.
I just checked out the St. Louis Post Dispatch on microfilm and found this. The newspaper mentioned that the 'Yes' concert for Kiel Auditorium April 17th 73 quickly sold out after the tickets were placed on sale. Another show was put on sale and that one almost sold out. That show was also at Kiel Auditorium on April 16th 73.