What frameworks do you rely on when creating messaging and positioning? How might these frameworks differ if creating messaging and positioning from scratch vs. updating existing messaging and positioning?

There are a lot of frameworks and approaches to positioning and messaging, but I’ve found that a simple document including the following elements is most effective:

• Tell the story: Starting with a narrative acts as a forcing function to ensure you’ve thought through the positioning and can tie it all together in straightforward, customer-facing language. Amazon’s internal press release is a good framework to get started.
• What is it? A single sentence that describes the offering. No fluff. Keep it simple.
• Who is it for? Define your target audience or personas. Be as specific as possible and focus on who cares most, not just anyone who may find value in the offering.
• What problem does it solve? A short statement that frames the customer problem (the “from” state) aligned with the category POV. Include supporting statistics and data points whenever possible.
• How else might a customer solve this problem? Include competitors as well as workarounds, apathy, or the status quo.
• What are the key benefits? Focus on the top benefits that matter most to your target audience, not all of the benefits.
• How is it different? How the offering solves the problem better, differently, and/or more effectively than the alternatives.
• How can you prove this? Proof points or reasons to believe that support out your claims. These may be supporting features, data points, or customer quotes. 
• Core messaging elements: These are the “greatest hits” or the pre-approved statements that internal stakeholders will use verbatim. They may include a tagline, elevator pitch, etc. All other messaging is developed and tailored to audiences and channels using the positioning and messaging as guidance.

To the second part of your question, I don’t think it matters if you are starting from scratch or updating existing positioning and messaging, as long as you include these core elements.

We use a messaging ladder (a slightly customized version of many i've seen online) but are increasingly using a broader framework to connect our messaging to the buyer journey. The framework we use to structure the messaging doesn't dictate how often we update the messaging. For this, we try to anchor our positioning on our product vision at least 1 year out and figure out evolutions to our messaging that help us realize that long-term positioning.

There are a few key components of messaging that are crucial, no matter what framework you are using:

• Define the problem statement. What is happening in the market that necessitates your solution, and what are the business impacts?
• In an ideal world, what does the solution look like? This shouldn't mention your product at all, but instead should be more of a point of view. This is helpful when determining what your thought leadership should focus on.
• What is your solution? This includes short bites like the elevator pitch and positioning statement, but can also have longer form persona-based stories focused on more specific pain points, solutions, and benefits. This is where feature mapping can also be helpful. 
• How are you different from the competition? I find it useful to map competitive messaging to our value props. Make it easy to see where that white space exists and guide how to present that message.

I have relied on the Message House and the Positioning Canvas (from April Dunford's book as well as PMA) for my work in the PMM field. These frameworks are quite well-known and there are a lot of examples available on the internet too.

They do not differ as such when it comes to creating something from scratch or updating existing ones. These frameworks give you a structured thinking process that you go through when filling it up. So that you can arrive at a core value prop that is backed by facts and features that are grounded in solving real customer problems. And these components do not change whether you are creating from scratch or updating.