I have never ever seen someone be able to successfully change their extension date 😕
About all you could do is when you go for the next yearly extension is ask them IF they'll give you less than a year (so that it will expire on the 15 of Oct (shorting you almost 2
***
months on the year) <- That is the ONLY way it might work and still it's iffy..
Other than that you do as Robert suggested and exit/re-enter without a re-entry permit and start all over from zero