I wonder if the price of the new lens will increase the value of mine, or if I should consider selling it before it loses too much value
Well, a few things.
1. The new lens doesn't actually exist. Canon's history for the past year would indicate that by the time they announce the lens, you have a 6ish month waiting period for it to actually arrive at your door, and the possibly return it because something is wrong to get fixed. Even if you avoid the latter part, it's 2013 before you can get it. Which leads to 2:
2. You can use the current 35L and take pictures. That has monetary value, especially if you are a pro who makes money from your camera. Even if you are not, the ability to take quality photos at 35mm doesn't go away cause Canon might release a new version.
3. All lens prices are doubling, not just Canon's. The original 35L is retailing at $1479, which means the new one is probably $2k+. If it's like the 24-70 market, it'll only increase the value of your lens. For example, you paid as much used as what someone would have paid for a new 35L on its release: http://www.canonpricewatch.com/product/00026/Canon-EF-35mm-f1.4L-USM-price.html
All those combine to say that, unless you never use your 35L, you should keep it. And if they end up releasing the new 35L at $1500 and it hurts the value of your current lens, well, consider it a blessing that Canon released the lens you want at a reasonable price