First of all, whether or not converting to lower bit-rate will be noticable, is only for you - your ears to decide!!
Second, i have an iPod video, and cannot fit all my music on due to my wish for higher bit-rate so... if sound quality is ALWAYS a must, then i would advise you do not reconvert to anything lower than 192kbps. The difference between 256 or 192, especially on an mp3 player, is not so much different (also depending on your earphones). So, its your choice - whether you really must have high quality music on the go!!
I have an ipod shuffle, but i was thinking on getting a Creative Zen V Plus. The problem is that i have a big music library from itunes and i am afraid of losing sound quality; even more, now that it sounds Excellent at 256Kbps. What do you guys think?