This happened to my friend earlier this and they just fined him 500 baht one day overstay but as others have said go to your local office and ask. Curious as to what extension you think requires 500k
Only amphetamine and morphine based drugs need a prescription. Most stuff is available over the counter and the prices are cheap. Most GPs will give you a prescription for 3 months supply if you tell them why. In the very unlikely event of you getting stopped on entry make sure you have a copy of your prescription.
I take it that you have a re-entry permit? I can't see any reason to transfer it as you're not renewing. The only problem is the IO on exit may make a fuss.