Question: Working on a website redesign. We have documentation from workshops that give us insight into the pain points of the existing site from the clients and their business requirements. How useful would conducting a heuristic evaluation on the existing site be if we are planning to redesign the whole site? I was thinking initially the learnings from the existing site would help me design the new site but I'm questioning how valuable doing a heuristic is?

Answer: 
In this case it would probably depend on how deep you want to go into the existing issues with the website. 

Sometimes clients and businesses' requirements are not enough, as they might refer to some particular problems, but not to the underlying structure. 

Having said this, I would probably invest my time/resources on the new version instead. You could do some tests before the relaunch, making sure everything is easy to understand and interact with.
I would restate the question slightly: do you learn enough about what improvements to make, as well as what you're currently doing well and should continue doing, such that it's worth the opportunity cost of doing it?

I would say that it depends on what you already know from these workshops and whether you feel like you have some open questions or an insufficient understanding of the strengths and weaknesses of the website. One quick way to answer this is to do a very fast heuristic evaluation: in 20 minutes, go through the existing site and see what you learn. If you don't come across anything that you didn't already know from the existing documentation, you can probably stop; if you uncover new issues that weren't already known, then you should continue. A 20-minute investment is small enough that it probably won't have a negative impact on the time that you would spend working on the new version.