Often times when I'm using field calculator in ArcGIS Pro, I'm starting off with a previously-used calculation. But when I try to go replace the field to calc, the normal workflow I'm used to doesn't work. Typically when editing text (including in ArcMap), you highlight text to replace, and add the next text - immediately deleting the highlighted text in the process.
This works fine if you're typing the new text, but when you go to double-click on a new field, for some reason the software just adds that field in front of or behind the highlighted text. See the screenshots below:
Highlighting !Zoning! to replace it with the LU_2030 field value:
Double-click on the field, and Pro just adds it in front of !Zoning!, without deleting the old field value.