Expats are not that significant in Thailand to make any dent in your so called "expat scene". Firstly, expats are such a small slice of population here that no one will notice anything. If people want to come to thailand pre or post DTV, they will find a way. There is always a way. Yes, you might live on Sukhumvit and only have expat friends so I don't know how much integration you have actually done. I don't notice any difference between high and low season because I have 0 to do with any "expat scene". I do not believe that the DTV is going to make any noticeable difference at all except if you only spend time in expat bubbles, perhaps you might see a few more white faces around Won't that be something that you enjoy though?
Yes, you need to cancel your visa. You can't have 2 visas at the same time. Just because the embassy issues your visa, you might have trouble at immigration when you enter OR when you try to extend. Just cancel the visa before you leave.
Just an FYI for anyone wanting to go to HCMC. They have no appointments until Sept 10 as of today. You will need to book well in advance for this embassy. I got one on the 16th and there are plenty during that week. It seems there is only 1 appointment available for each 10 minute slot. Don't wait until the last minute, you might be disappointed.