I seriously doubt you will have problems, and if you are only modifying the CSS, then you should just be able to re-download the WordPress.com version of the theme and replace all of the PHP files to fix any problems that might crop up in the future.
This sort of theme is fairly simple. There's not a whole lot going on under the hood, so I wouldn't expect much in the way of future problems, at least not for a few years anyway. WordPress is pretty good at providing backwards compatibility and you need to do something pretty dumb ass for it to break on an upgrade - or complex, which this theme is not.
There's potentially a depreciated function or two in there, but that's likely to cause a catastrophic crash in there if it gets removed from core WordPress and so it will tell you which file and line the problem is. Then you just need to download a fresh copy and either copy ALL the files over like I suggested above or you could find the changes in the new version and apply them to yours. Either way, any problems should be simple to fix I would suspect.