How to exchange/remove/overwrite a PDF's background?

as the title says I want to change background of a PDF. This are some slides, and to save toner I thought it would be good to change the background to white. I tried pdftk with its operations: background and stamp, but the don't work as expected:

pdftk old.pdf background white.pdf output out.pdf
pdftk old.pdf stamp white.pdf output out.pdf

I also tried convert (bloating my pdf upto 200MB) as done this way:

convert old.pdf -background white new.pdf

with no luck. Do you know a way I could perform this?

To deal with these presentations, I used a few times Sun PDF Import Extension. When installed, you are able to import PDF presentations with OpenOffice.org Impress. Even though the conversion is not 100% smooth, you are able to edit the background and change the color. It may be a bit tedious if you are dealing with lots of slides, but it's the best solution I found for these cases. You should note that everything is converted into Impress' objects.