I accidentally "upgraded" my account to non trial. I'm still using the trial and needing to test stuff while still in the developing period. The credit card I put is not the credit card that will be when I upgrade my account intentionally. So in short words, I need to change my status back to "trial" so Google won't accidentally charge me even though I still get the trial privileges.
You cannot change back from paid to trial, however this is not necessary. The remaining credit from the trail will be applied to your account, so you will get just as much for free.
If you don't want to be charged you can disable billing for your project (although this will disable many services).