I'm looking to upgrade cameras not just because the 10D is getting very old but it's starting to limit my photos. For example last week I was shooting a film festival inside a theatre and the only way to get the shutter speed low enough (on all of my lenses) was to turn the ISO up to at least 800 which is one step lower than the max.
just want to make sure you know that just because ISO 800 is one step below the max doesn't mean anything by itself. plenty of cameras perform well all the way up to the maximum native ISO.
As for upgrading to an older model I figure that since I plan on keeping it for a long time I may as well go for the latest and greatest with all the bells and whistles like video.
there's something to be said about buying older bodies used and just upgrading a little more frequently to newer used bodies...but if you're set on a new one, there's nothing wrong with that.
As for lenses I probably won't be selling any of mine any time soon because they have a good range covered from wide angle to telephoto. As for a 70-200 I'm not sure how much that would help, I've never used one before although I might be able to borrow one and find out.
I'm not sure you quite understand the significance of lenses. all the 70-200s are constant f-stop (f/4 or f/2.8), whereas your 100-300 is variable f/4 to f/5.6, and they are much better optically - resolution, less distortion, less flare.... the 100-300 is also the weakness in your lens lineup, though I guess if you don't use it very often, it's not a big deal.
if you're set on not rearranging your lenses, you should probably switch to a 5D. but again, try one out first, because it's much less responsive than an APS-C camera, and some people have a problem with that.