How to use 'convert' to change several from png to jpg?

[joe at actionline.com]

I use the following script to convert a batch of images to a uniform width
of 900 pixels.  It works fine ... but how do I need to modify it to do the
same task but also convert all the originals that are .png files to .jpg?

mkdir 900
find -type f -maxdepth 1 -print0 | xargs -r -0 -ixxx convert -resize 900
-quality 90% xxx ./900/xxx