Since the Numbers '08 and Numbers '09 Dock icons were identical, it can be difficult to keep track of what you are using. Numbers '13 has a different icon, featuring a Bar Chart with 4 bars instead of 3, but you have to look carefully.
A Numbers '08 document will open in Numbers '09 and when Saved will become a Numbers '09 document, by default. A Numbers '09 document will open in Numbers '13 and when Saved will become a Numbers '13 document. Numbers '13 will not open a Numbers '08 document. If you have all three versions installed, there is no problem. Inconvenient maybe, but not a tragedy.
"It tells me to open in 08 and save to 09... How do I get the 08 docs to save in 09?"
I doubt very much that this is the message presented. Like most applications, an earlier version of the application cannot save in a file format that didn't exist when the application was written. Development of Numbers '08 stopped when Numbers '09 (and the file format for Numbers '09) was introduced.
Later versions of most applicartions, Numbers included, are usually able to open (and save to) the file format of at least the most recent previous version. Numbers '09 can open Numbers '08 files, and can save to the file format used by Numbers '08. Numbers 3 (aka Numbers '13) can open Numbers '09 files, and can save/export to the file format used by Numbers '09.
To save a Numbers '08 document in Numbers '09 format:
- Open the document using Numbers '09
- Save the document as a Numbers '09 document.