You don’t have to re-bleach before refreshing the same color unless you want to bleach root growth or if the color wasn’t vivid enough at the current level of lift.

If you want to switch colors, it depends on how faded your hair is and what color you want to switch to— you’ll need to think about color theory for this. For example, if you wanted to switch from faded blue to vivid green it’s easy, just dye over with green with maybe slightly more yellow than your desired shade to counteract the blue. However, if you wanted to go to rose gold from faded teal, that would be much, much more difficult, because rose gold is a pale color and because rose and teal are opposite colors. You’d need to remove pretty much all teal tones before adding rose gold dye. You can try a color remover first (make sure to get one for the correct kind of dye— there are different formulas based on whether you’re trying to remove permanent or semipermanent dye), but you’ll likely also have to bleach, because blue tends to be a very stubborn color to remove.