Even switching to free is not enough to gain serious worldwide exposure. It helps, but there's also an element of chance and just getting it in front of the right people.
My current app hit #1 in a fairly major category (music) quite easily when we switched to free, but in the rest of the world it barely cracked into the top 50 before immediately sinking again, mostly because no one knows it exists. Still trying.
I did release a 'lite' version with fewer levels, but I do have plans to put together an ad-supported 'free' version. I'll follow up with results when that all goes down.
Regardless, I stopped promoting my best selling apps and they sell exactly as many copies.