Originally Posted by Wautier

Personally; I've only had positive results.

But, do note the following:
- I always code my own WordPress theme, and have a clean HTML structure, meaning that there will be absolutely no bloat whatsoever.
- Free (or paid) WordPress themes will have bloat that you need to remove.

I've had friends who more or less followed the advice, but still ended losing traffic initially, because they messed up on something like: they had a different amount of related posts in the new theme, or a different display (meaning, maybe they were listing related posts as thumbs with text in a previous version, and in a new one with text as alt's, etc) and all of those things matter quite a bit.

If you are certain that you made a clone of the page's contents on the new theme, that it uses the same exact titles, descriptions, etc, etc. and if it also loads faster, then it's practically a no-brainer, and you should do it ASAP.