You can copy purchases from your phone to your computer's iTunes library via File > Devices > Transfer Purchases so that you can take your own backup of them and be able to resync them back to your phone.
Also music (and films, apps etc) are not guaranteed to remain in the store, the rights-holder could remove them at any time (and if you move country won't be redownloadable) so if not already doing so you should be taking your own backup copy of them as well - you are only guaranteed one download of any item.