Should I change "Fixed" or "QC" items to "FTQ" once the error is corrected?

No. 

"FTQ" means First Time Quality, or it was done correctly the first time. "QC" or "Fixed" means the item was corrected through "Quality Control." It's important to keep track of these items so you can find recurring issues or trends in your quality performance.

By clicking "Fixed" or "QC" on an inspection, you are confirming that the issue was completed.