Q: I've just changed my Apple ID so therefore would like to gain access to my music in ITunes .upon tryin it says that I must wait 90 days to access my old iTunes account

This article - iTunes Store: Associating a device or computer to your Apple ID - discusses the 90 period

CCC

Sure, you can buy new things.  You just can't re-download past purchases for 90 days after changing the AppleID used with a computer, as the link provided by ChitlinsCC discusses.  To avoid this problem make sure you always have backups of all purchases you make which is a good idea in any case in case a distributor pulls an item from the store whereby you won't be able to redownload it.