The code depicts the text. If the text is absolutely identical and the code delivers the page in raw format the exact identical way, then it would not affect it.
If you changed the order of text delivery in raw format, ie. right column to left column, and the code literally shifted before to after the main body, then you have now changed the meaning of that page according to Googles mathematical algorithm.
If you add new words, etc, a slight few words missing or few additional words, may be all that is required to change the meaning of the page to a mathematical algorithm.
What is the outcome of such changes? Well, nobody ever has the answer until you actually make the changes and observe the results. This is why once I used to do this on client sites, I would warn them about initial fluctuations. After that, very minor tweaks where done to test page by page basis, whether the page went up or down, or revert the page back again, etc.
This was all lovely when one could just make a landing page that could fool Google, but it's not that simple now.
If nothing other than a theme change was done, yet the theme reorganised the complete layout of the page, that could change and produce fluctuations.
The beauty with XF is that it uses HTML5, which Google will completely drop specific elements out of their equation and use only body content, ie. footer, nav, sidebar, etc... HTML5 allows for the clear and definitive isolation of such areas for Google to dismiss, along with browsers identify content for inbuilt readers, etc, ie. Safari.
If your existing WP theme is HTML5 and your new one HTML5, then it may not make a difference. But if you are upgrading to HTML5, you may notice slight fluctuations, depending on how much content exists.
Google is pretty smart, and even without HTML5 it can identify certain areas based on how the information is portrayed to them, ie. a massive long bullet list of word / few words linked, is likely navigation, thus they dismiss it. Footer location content, dismissed, etc. But it isn't as good as how they manage HTML5 due to specific tags existing to denote such areas, regardless where they lay in the page layout in raw format.
A few pages doesn't mean a thing... meaning, 100 is even a few in the scheme of radically affecting traffic when talking blogging / forums vs. static websites which are easier to manipulate and control changes.