You can import masters from another InDesign document (any version) into the active document. If your destination document contains master pages that have different names from any master page in the source document, those pages and their document page overrides will be unchanged.
Choose Replace Master Pages if you want the masters from the source to override the destination document’s masters with the same names. If your destination document does not have any overridden items, it is safe to Replace Master Pages on import.
Choose Rename Master Pages to change the page prefixes to the next available letter in the alphabet.
Once you have imported masters from a source document, a link is set up between the source document and the destination document. When you subsequently load masters from the same source document, the association between overridden items and their parent items on reloaded master pages is maintained. This association lets you keep master pages in different documents consistent without putting those documents into a book.
If you want to use this method of keeping master pages consistent, you should load the master pages from the source document before overriding any objects on the master. If your document has overridden items and you have never imported masters from any source, those overridden items become detached the first time you load from a source document and replace master pages with the same name as the parent master of the overridden items.
If you subsequently import masters from a different source document, however, and choose Replace Master Pages, the overridden items may become detached. Any same-named masters from the new source document will be applied to the document page containing overridden items, creating two sets of objects.