How to programmatically suggest changes to a text to match some other text?
Some time ago, I stumbled upon the Author Obfuscation task of PAN16 workshop. The task at hand is given a document, paraphrase it so that its writing style does not match that of its original author.

Although, it's not exactly what you want, you can take a look at their proceedings and find useful tips regarding authors style and how to "change" it (i.e. in your case, an "author" is a collection of documents from the same source).

For example, one approach
denotes specific metrics that define an author's style (average sentence word count, use of stopwords, pos-type frequencies). You could use the target's scores in these metrics as your gold standard and try to modify the source text, in ways discussed in the proceedings of the workshop, so as to mask your source text to match the target one (in terms of these metrics).