I know I can open my QGIS .dbf files in Excel (which converts them to .xlsx files). Can I then convert the edited file back into a .dbf file for use in my QGIS project? In other words, can I use the familiar functions, such as find & replace, in Excel, in order to edit my QGIS dbf files?
Natively you can't. But there are addons / plugins for excel to support .dbf format such as http://exceltodbf.sourceforge.net/
Libre Office supports .dbf files natively.
However, I do not recommend editing .dbf files (connected to .shp) outside of a GIS. Also never edit column names or add or delete rows unless you really know what you are doing there.
I prefer using .csv files instead. You can open them with excel, use excel functions and all that stuff (but not saving formulas as .csv is a text file format).
The recent Excel version could open/read dbf but no longer save to dbf file.
Anyway if you have an older excel version or use other tool to convert back to dbf be very carefull trying to edit shapefile data this way. You easily could corrupt your data (the link between the .shp (geometry) and .dbf (attribute table) repose only on the order of data and not on any ID/key) so reordering or sorting the dbf will result in corrupt shapefile where data and geometry didn't match anymore.