Hi Kivahan,
Since I was involved with your previous question. I can try to help you here as well.
Normally, you’d convert the Field Type instead of just changing it. But we established you don’t have this option in 16.5. If you simply change the Field Type, the field value doesn’t change automatically (this is what the Convert To feature does). Imagine you have a 20 degree field with an angle definition. When you change the field type to Real Image Height. OpticStudio simply interprets the number 20 as now being lens units (most likely millimeters), and this might be too large for your optical system (as indicated by the warning).
All you need to do is reduce the field values in the Field Data Editor. The amount by which you should reduce it depends on the system. Before changing the field type, you could use the real chief ray landing coordinate (also shown in my answer to your previous question) as the new field value, and then change the field type to real image height.
Let me know if that makes sense.
Take care,
David