When changing encoding in Maven project (CP-1252 to UTF-8), how shall i handle XML-files with encoding iso-8859-1

When changing encoding in Maven project (CP-1252 to UTF-8), how shall i
handle XML-files with encoding iso-8859-1

I'm working with legacy code at the moment. The project is a big
maven-based project and one of the tasks is to change encoding, from
cp1252 to utf-8 .ie
<project.build.sourceEncoding>cp1252</project.build.sourceEncoding>
to
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
Most of the code are already utf-8, but there are some xml-files in there
that are not, (and have their header as <?xml version="1.0"
encoding="ISO-8859-1"?>.)
Do I have to manually change these files to utf-8 in their headers, or
will it work anyway with just changing the setting in Maven to UTF-8? I
guess that it would be a lot of possible corrupted characters by doing
that?