Some great comments here. Almost everyone seems to agree that moving up limits would be a mistake. However, I think this is a perfect time to move up limits. If it were me, I'd cash out a good chunk, probably keep a good portion in a Neteller account for reload bonuses. Then I'd move up one level. You say you have been playing $5-15 SNG's. I'd probably move up to $20 SNG's. I'd set a loss limit, say $100 or $200. If I lose 5 or 10 in a row then I'm probably out of my league. But If I can place in the money in a few of those first 5 or 10 then I'm probably at a level I can compete at. But I would definately make sure I stick to a loss limit. Even if you win or place in the first couple and then go on a losing streak, when you hit your loss limit, move back down.
The game does change at different limits. The problem with moving up limits and then back down is that you might have adapted your play to the higher limits, even subtely, and then you will find you aren't as successful at the lower limits because you have a slightly different play style. This is something that I've experienced. I compete at one level successfully and move up levels and don't have enough success so I go back down and I'm no longer as successful at the lower level since I've adapted my play. So it takes me some time to get back into the groove so to speak at the lower levels. Hope that makes sense, it made sense in my head, but that doesn't always mean much when it comes out!
A grand is likely not a life changing amount for you. Though it makes a pretty nice early Christmas present for just about anyone. So what you do really depends on what your goals with poker are. Do you want to just have some fun and a nice hobby that lets you make a little extra cash from time to time? Or do you want to be able to have a secondary income from your poker skills? If the first one fits you closer, then cash out the bulk and buy some nice stuff for your wife or your friends or yourself and stick to the limits that you are comfortable at and able to play at successfully. If the second one is more along your goals, then you are going to have to move up limits. This is the opportunity to move up without risking losing your shirt. In the end, it's your call, and whatever you decide, I'm quite certain it will be the right decision for you.
Congratulations and good luck with whichever path you choose!