Formula field popup warning message

Formula field popup warning message

I am getting a warning message before I add a new fx Formula field "All reports involving this field also will deleted Do you really want to change expression?"

I am confused because this is a new field and I have not seen this message before. I don't want to lose any reports. Can anyone explain why I am seeing this? I have not seen before.

formula ((field1/field2)*100).round(2)

Thanks