You don't really need to worry about the bundle portion, if you upgrade your data portion to a product that is eligible for bundling, then it will move to a new bundle automatically. Bear in mind that pro-rata fees are calculated on the full product value and not the bundle value.
You will not lose any functionality will not change as well as your username, you will simply receive more data. Just remember that you do not need to bundle and unbundle. As long as it's an eligible Capped product, you will remain in bundle. Check our website if you want to be sure that you are selecting an eligible product, otherwise you should receive a warning that the product you've selected will result in you being unbundled.