There are plenty of source code control tools... but that was not your question. No, there is no merge prior to LV8.5. I would have two questions for you. Since you appear to have 8.5, why not upgrade your project? You are talking about making changes anyway (I assume this since you want to merge code), if you can not upgrade the project, you could merge your changes in 8.5 and save for prior version... but what a pain!
As far as source code control, MS source safe works well in many environments, but support is limited. I have used perforce, subversion (free), and clear case. They all have their pros and cons.
Good luck.
Paul