When we inherited this code base, it had a WordPress site intermingled with a CodeIgniter site (called the partner portal). It quickly became apparent that we should separate the two, which we did.
We moved the WordPress site to wpEngine and left the partner site on the existing server. We also implemented several new features and a couple bug fixes on the partner portal.
One item of note was that we found an mitigated evidence that part of the site had been hacked. Our research found no evidence that the malicious code exposed any sensitive information, but we removed the code immediately.