I did this from a 9300 too. You get the option to "replace" or "merge". Whenever I do this, I know my new phone is blank, so I always choose "merge" to be on the safe side!

The only things you can't transfer is 9300 notes, and I had a TON of those. The only way I found to do it was to copy all the notes to a PC (via the memory card), run the STRINGS command on them (google this, you can get it from Microsoft) which will pull all the readable text from the files.

Convert all the files in one go, something like (on the PC, in a CMD prompt):
CD directory-where-notes-files-are
FOR %F IN (*.) DO STRINGS %F > %F.TXT
This is from memory so you may have to check the syntax of the FOR command (google!)

You can can then copy the resulting text files (*.TXT) back to the N900. They all end up with a stupid "Copyright Microsoft blah blah blah" header but at least they are readable.