Not personally, but I know that people sometimes are not happy with the mantra they are using.
First things first, what are you not happy with your mantra? Is it because of the TM movement itself? or is it because you associate something negative with the mantra? or you just have trouble saying that particular mantra to yourself?
If it's because of the TM movement or something other that is negatively associated, why not keep using the same mantra, but come to recognise the negative feelings you are having and actively choose to let those feelings go as a part of your meditation.
Remember, the mantra is just a tool for initial focus to help you detract from other thoughts and assist in you getting deeper into the state of meditation. It doesn't need any meaning or associations attached to it, but if you find there are, You have the choice to let those things go.
Saying that though, if you really want to change your mantra, then by all means go for it. Find something you are comfortable with.... then let that comfort go.
Hugs
Giles