Creating a brand isn't easy, but what's much harder is making sure a company can 'stay on brand'.
After working with startups for many years we've seen how the brands we create tends to loose focus if there isn't someone specifically tending to it. You really need to keep working on the brand, thinking about it, evolving it... or it looses focus and starts to come apart. That's what brand strategy is.
Great companies pivot, great branding never pivot
Early stage startups need to change quickly and take big chances. Pivoting is something that's taught and encouraged, but it's one thing to pivot product and another thing to pivot brand. If a company is branded properly, they should never have to rebrand. Brands can evolve and get 'brand refreshes' but you probably shouldn't be changing your name or your logo unless you didn't quite get it right the first time around.
Here's a quote from Paula Scher that every designer should read:
The designer needs to be ever present because, inevitably, at some side meeting, something will be suggested that will totally destroy the form of the logo. Something can be suggested innocently, with the best of intentions, that will scuttle all plans, compromise all standards, and destroy the integrity of the design. The only person who can know this and stop this is the designer.
Core values don't change, the spirit of the company surely should not change. The visual identity is a manifestation of the spirit of the company, so it stands to reason that the look and feel of the company as a whole (when done properly from the beginning) should never have to change, even while everything else is pivoting and shifting around it.
Think of the big brands that have re-invented themselves, apple, nike... The one thing that stays consistent is the core values of the brand, the visual identity also stays remarkably in-tact over the course of many years with minor tweaks to improve upon it.
Don't forget your brand promise
There's a million decisions made every day in a fast moving startup. Something has to be the rock that holds it all together. That thing is the core values. Another translation fo the core values is the brand promise. It's another way to get across what distinguishes a company. As things get shifty, make sure you hold true to the values and deliver on the promise and everything will be just fine.
What about successful re-brands
Re-branding can be a good thing, but only when it's done for a good reason. Lots of great startups outgrow their branding, but a great startup won't ever outgrow great branding, the branding exercise is an exercise in figuring out who the company is and why they exist.
Use a brand guide and maybe also a style guide
Instead of going into a lot of detail on this one, just read this article on what the difference is between a style guide and a brand guide.