Given that you said you'd taken the existing bike on finance I assume that means you'd be financing the extra 1700 to upgrade to the pre-reg SV650S? If that's the case then I'd say it wasn't a good move just now. You'll likely be losing out on the deal - and you're not in a hurry anyway since you're not doing the DAS until June. If you were doing your test tomorrow my advice would probably be different. It also means you're going to paying finance on a bike you can't ride and that is going to feel painful.
You have three months to try and sell your bike and if it's worth more than £2300 then you could make money there. Dealers sitting on pre-reg 2011 SV650s will bite people's hands off for cash, more so than for other bikes and the only risk there is there won't be any more left in June. The best way to mitigate that risk is shop around the country on ebay or whatever and see how many pre-reg or very nearly new SVs you can find around that price. If there are a lot (and I suspect there may be), then you don't have too much risk here either.
It's all a balancing game on whether you feel the risk is too great to wait, but be very cautious of jumping into more finance if you have to wait till June to do your DAS.
I also think there may be other reasons why the bike is not selling, looking at your advert. You're trying to give people too much information on your advert and I think that may put them off - remember you are trying to make a sale, not give everyone all the information in the world. I'm not saying you should lie, but let them ask some of the questions.
Also - in your advert you include a video of you riding the bike, where there is a lot of wind noise in the video (I only flicked through it). I'm not sure what the point in the video is (it's interesting to us as vloggers, but not a buyer of the bike). If you did a static walk around of the bike showing that it was undamaged and all nice and shiny after a wash it'd provide MUCH more value to your sale than the one you've included now.
Sorry if I've sounded harsh, but I'm just trying to help you get the best value from your biking.