Collections in DSpace are not mutually exclusive and items can be included in more that one collection, however there can only be one primary collection. The primary collection is selected at the point of record creation, inclusion in subsequent collections is done through mapping. The primary collection can be reassigned, and collection mapping can be changed, but the process for doing each is different.
Below is a description of how to change the primary collection for an item.
Mapping is DSpace 7 done when editing an item.