Namespaces were added to XML 1.0 after the fact, but care was taken to ensure backwards compatibility.
An XML 1.0 parser that does not know about namespaces will most likely not have any troubles reading a document that uses namespaces.
A namespace aware parser also checks to see that all prefixes are mapped to URIs. Otherwise it behaves almost exactly like a non-namespace aware parser.
Other software that sits on top of the raw XML parser, an XSLT engine for example, may treat elements differently depending on what namespace they belong to. However, the XML parser itself mostly doesn't care as long as all well- formedness and namespace constraints are met.
A possible exception occurs in the unlikely event that elements with different prefixes belong to the same namespace or elements with the same prefix belong to different namespaces
Many parsers have the option of whether to report namespace violations so that you can turn namespace processing on or off as you see fit.