I'm too lazy to write an XSLT stylesheet, but that could be a solution.

e.g.: see my previous answer for this question

I recently sent this question to the NCBI help desk directly, and this is the response I got.

Hello,

There are some conversion utilities on the Gene FTP site:

ftp://ftp.ncbi.nih.gov/gene/tools/

See the README there for details.

However, the conversion from the NCBI XML to GFF3 didn't seem at all straightforward. Since it appears that I suffer from the same laziness issue that Pierre has, I ended up finding another way to get the data I needed without converting from the XML.