What is the most secure encryption algorithm that keeps the entropy the same or even lowers it? On top of my mind comes caesar cipher, but that is not really secure

so what is the most secure encryption algorithm that doesn't affect entropy or even lowers it? 

I know this might not be as secure as normal encryptions, but I'm asking about what is the most secure among these types of encryptions?

basically I have a series of bytes, and I want to encrypt them in a way that a machine that only checks the entropy to detect encryption gets bypassed. so basically this encryption doesn't affect the entropy, or even better if possible, lowers it. 

I thought maybe I can just replace each byte with another, but was wondering is there any better method? keep in mind that security is my second priority and keeping or lowering entropy is my number one priority
This seems pretty simple if your 'isEncrypted' distinguisher looks for a cipher text entropy rate in excess of 7 bits byte. But it's not exactly an encryption. Just hex encode it.

All good cipher text is computationally indistinguishable from perfectly pseudo random.  Take the most securely encrypted cipher text from any decent algorithm.  It will be 8 bits/byte if occupying all the bits within a byte.  Then hex encode it to ASCII base 16. You now need two bytes to encode any single original byte of cipher text. Ergo the entropy rate has been halved to only 4 bits/byte and you still maintain maximum security. Your message would of course get correspondingly longer.  

In extremis, go to UTF-32 binary encoding which could just be a series of two really funky characters.   Perhaps two Egyptian Hieroglyphs of your choice, each occupying 4 bytes. Mean entropy rate = 0.25 bits/byte.