If lyx2lyx is asked to convert a file to the format it already is,

If lyx2lyx is asked to convert a file to the format it already is,
then just perform a null conversion. This seems better than aborting
with an error. I can imagine a script that converted all files in a
certain directory to format 498, and you don't want it to be an error
if one of them is already 498.

Prior to this, what lyx2lyx would do is convert to the lowest format
in the current version. So, e.g., asked to convert 498 to 498 it would
convert to 474. That is obviously wrong.