Odd, I almost want to ask how old all the respondants are to this query. I don't mean that in an insulting way. I am just wondering if things are being taught differently these days. I was actually taught that you ALWAYS maintain code in the original style and not a single person made such a strong statement. Some mentioned a personal preference or even conditional preference, but not having been taught that coding in the orignal style was the rule of maintenance programming.
While doing so can be painful, I have actually experienced the pain of breaking this rule also. Breaking the rule is worse.
There are two outcomes of breaking the rule. The first and most obvious is everyone does and the resulting code is so abysmally horrendous it starts to rot before your eyes.
The second is a complete, elegent solution coming as close to beauty as you can imagine. You're co-workers will likely crucify you for it. The reasons for each will be different, the result will be the same. Your efforts will NOT be appreciated.
When dealing with old code that is no longer representative of current coding styles, changing the coding style should be an agreed upon project or it should be left as is. Mixing it up or fixing it unauthorized will never work in your favor.
Someone above mentioned the pain of reorganizing someone else's play pen. Believe it...
If you feel you have lost yourself, create a personal project outside of work for personal development. Your employer doesn't care about your being creativly restrained. Your employer wants you to be replaceable. Code only you can maintain restrains your manager. Code your co-workers have to be trained to understand costs your manager. Code that looks like all the rest makes your manager comfortable and is what they believe they are paying you for.