Is the native format used by Word Online the same as any Standard Word Desktop format (i.e., if I upload will it prevent it from having to be converted for editing in Word Online or Word for iPad)?
After some investigation and no other answers, the answer seems to be the latest version of Word .docx.
I opened an old file and saved it as .docx and it opened on Word Online without conversion.
For those of you interested in the code. If you are converting files you need to use the new SaveAs2 function on the document in VBA for the compatibility features to be correct so that it won't require conversion.
I wrote an application DocTo which can batch convert documents, which may be useful in this scenario.
Something like this
docto -f "C:\Folder\With\OldFiles" -O "C:\Folder\For\NewFiles" -T wdFormatDocumentDefault -C 65535
The -C 65535 ensures compatibility with the latest version of word.