Tiff file problem

I scanned some photos wayback using PS6 and they were saved as tiff's.
I recently opened them in PS8 no problem but wanted to save them as jpg for
web use.
I "saved as" xxx.jpg but when I went to open them error message came "not
file type"
On looking at files they still had tiff extention. I could open them if I
chose "open as" jpg, I always assumed that "saving as" created a new file
and left the original
How can I get round this problem or is to late as it would appear the
original file have be over-written.