It depends on what you're doing. If you're using Direct3D, the only reason to switch is to gain access to hardware features that you can't get otherwise.
For a 2D game, it is theoretically possible that certain features might interest you. For example, OpenCL or DirectCompute. Civilization V gets some use out of DirectCompute for decompressing textures. This dramatically reduces load time, because it only has to load compressed data. And 2D games often have a lot of images. If load time is an issue, this might help.
OpneCL should be available on all GeForce 8xxx hardware and above. On ATI, it's available on HD 4xxx and better hardware.
I'd say the biggest feature you might use in DX11 is the ability to more arbitrarily read and write images in shaders. That could be useful for certain specialized effects. I can't think of any right now... which should tell you how likely this is to be important.
In general, I would say that if you're using D3D, you probably won't gain much. And if you don't already understand what the new features are and want to use them in your game, you almost certainly won't gain much. Most of them are going to be more complex.
OpenGL is different for one major reason. OpenGL 2.1 (the more-or-less equivalent to D3D9) sucks as an API. OpenGL 3.3 (plus some more recent extensions that run on 3.3 hardware) improves the API a lot. Explicit attribute location, separation of shader programs, everything in shading language pack 420, and so on.
Some of these work well enough in extension form against 2.1. Some of them... don't. I'm still not sure if it's OK to use explicit attribute locations with
attribute declarations (before the current
in syntax). Shading language pack 420 specifically says that it doesn't work with GLSL versions before 1.30 (aka: OpenGL 3.0).
That's not to say that GL 2.1 is unusable. It's certainly functional and serviceable, just not enjoyable. Though some drivers that you'll find that implement it aren't serviceable. ATI's DX9 hardware is no longer being supported, so those drivers are really buggy. And Intel never cared about OpenGL, so using shaders on their hardware is pot-luck.
But personally, if your intended user-base allows it, I would push to GL 3.3 if you're using OpenGL. If not, you'll just have to deal with the 2.1 issues.