If you are planning to migrate your SCCM 2007 environment to SCCM 2012,you may have to think about your packages,collections,OSD and other stuff.
you cannot migrate all collections from SCCM 2007 to SCCM 2012.Couple of things to note before you migrate them.
Points to note on collection migration:
1. Collection can contain only either devices or users but not both.
2. Every collection must have Limitation.
3. Sub collections are no longer exists instead you have folders.
blogged couple of posts on archiving/removing packages not been used for X days that will help you to not migrate them to CM12.
In this blog,will show you how to identify the collections that has both users and computers member of it to look at the collections if you want to take of users/Computers query before you go on migration.
fcm.CollectionID 'Collection ID',COLL.Name
v_FullCollectionMembership fcm,v_Collection COLL
count(distinct fcm.ResourceType) > 1