Standard DRM copy protected songs are only available in 128 kbps and there have never been any plans announced to increase the bit rate on these. For some songs you can get iTunes Plus versions. iTunes Plus refers to songs and music videos available in 256 kbps AAC encoding and without digital rights management (DRM). There are no burn limits and iTunes Plus music will play without being converted on iPods, Mac or Windows computers and on other music players that use the AAC format.
Even if you were to convert the 128 kbps songs to 192 kbps MP3 you wouldn't get any increase in quality. When a file is compressed from it's original uncompressed format information is lost in the process, that information can't be added back by converting again. In fact converting from one lossy format to another can result in even more quality loss. It's a bit like photocopying a photocopy, the farther you get away from the original the lower the quality
