First, there is no such thing as changing VPN to APN. They are two entirely different things. The only way to change APN settings on a non-jailbroken phone is to perform a SIM swap, because the stock Straight Talk or AT&T SIMS do not allow you to edit all of the APN settings.
I was able to do this myself, so it is not impossible, but it is time consuming and not something everyone will want to deal with. Here is my topic on the steps I took to make it happen:viewtopic.php?f=6792&t=67564&start=10#p177462