I want to edit existing text on several fields of text and then save as a PNG file. Can this be done from the command line? If so can someone provide a command line example?

svg is nothing more than a text file so you can use classical tools (such as sed) to change the text
http://www.grymoire.com/Unix/Sed.html#uh-1

sed s/day/night/ <my.svg >myedited.svg
inkscape --export-png=myedited.png myedited.svg

sed (and other unix tools) has been porte to window, you'll find it @ http://unxutils.sourceforge.net/