I recently switched jobs and I want my old "from" address to now show my personal internet address (xxx@yahoo.com). I changed my "from" addresss on the webclient but when I send mail it still shows that the note came from my old company address. One more clue, my "reply to" address is correct now "xxx@yahoo.com" although you only see that when you hit "reply".

So, I need to get the "From" changed. Where is that stored?

I do not have either Notes or Outlook and am using Yahoo mail. Is this a matter of synching post my former Notes mail account? I do not have any mail client other than Yahool over browser. Is this the source of my issue?

Try changing your name in the owner setting in options.
I believe this is where the from name is derived when sending from the device.