logging out of old Apple ID into new one I know that if I log out of my old apple id and sign into my new one that I will still keep my apps, but i'll need the password to update each app belonging to my old Apple ID. 

but i'm just wondering if i don't want to do that, should i delete all of the apps of my phone and re-download using my new Apple ID to save the hastle?

also, i don't want to re-buy all of my music—will that remain on my device?  I do not want to waste unnessisary money. 

thanks!!
Anything purchased under the existing ID will remain on the device but as you note, you will have to switch logins to update those apps. Whether the hassle is worth it is something only you can answer. Obviously for free apps, you probably want to download them with the new ID.

Music will remain and since there are no updates, you don't need to remain logged in to that account.

When/if you upgrade to a new device, you will be prompted for the username and password that was used to purchase songs (videos, books, apps) in order to authorize the new device to play/use them.

As a side note, if you set up a new Apple ID because your email address changed, in the future, don't do that. Simply login at appleid.apple.com and you can change the email address associated with your account, which also changes the username. For the vast majority of people, one Apple ID is all you will ever need.