Does it really matter what the actual variable name is?
Perhaps you have a variable named 123 and another named abc. But the value contained inside the variable is really what counts. So if you need both variables to represent the same value, just configure them to contain the same value.
I agree with Rick that it is best to leave the two variable and assign the same values.
If you are forced to to have just one variable you would need to look at the code in a topic and then do a multifile find and replace to change the name. If you do go that route, then reate a backup first. I always recommend creating the backup as a zip file. That prevents you opening it accidentally and working on it before you realise it was the backup. This way you always have a clean backup that can be used to create a new clean copy time after time.
See www.grainge.org for RoboHelp and Authoring information
When you wrote that your searches weren't finding the instances of your UDV, what happens if you search for "rh-udv_start"? If you're using the search within RoboHelp, I noticed there is a "Find in Source Code" check box - is it selected when you perform a search that yields no results? One option would be to download and then use a tool called FAR (Find and Replace) to search your HTML code.
As far as whether you need to actually consolidate your UDVs, why did you set up a UDV in the first place? I'm presuming it was because you had a common string of text that was going to be used in many different places and you didn't want to make multiple changes - you wanted to maintain in one place. The primary benefit of using a UDV is exactly that: you only have to change it once. You don't have to search all of your HTML files to make a change. You're making your work so much easier because you've done the "hard work" and identified common strings of text in your documentation. It's a technique called "modular documentation" and when enforced, it can eliminate a lot of redundant work. That said, if multiple UDVs with the same definition makes sense, be sure to document your reasoning in your style guide as well as within a comment within the UDV itself.