Without knowing enough about your product and the actual changes, I can only offer brief guidance. This is not legal advice. Please consult an SME who can look into your documentation and processes.
Sounds like you'll have two separate products with different intended uses?
Your documentation should demonstrate you've done your due diligence to ensure each product is safe and effective to use by intended users, in the intended context, for the intended purpose. I assume you've done all that for the previous software. Even if you think the changes to the new software are minor, they need to be looked at in the same holistic way with safety and efficacy in mind. And then documented.
For example, are there any new, previously not considered users? Does the change in the GUI introduce new potential use errors? Or can the incorrect software version be accidentally released to unintended users?
What may be helpful, although not completely equivalent, is to look at what FDA considers important in the light of 510(k) submission for a software change (search for Deciding When to Submit a 510(k) for a Software Change to an Existing Device Guidance for Industry and Food and Drug Administration Staff).
In a nutshell, the analysis goes like this:
- Is the change made solely to strengthen cybersecurity and does not have any other impact on the software or device?
- Is the change made solely to return the system into specification of the most recently cleared device?
- What are the impacts of any changes to risks associated with use of the device and the impacts of any changes to the risk controls for the device?
- Does the change introduce a new risk or modify an existing risk that could result in significant harm and that is not effectively mitigated in the most recently cleared device?
- Does the change create or necessitate a new risk control measure or a modification of an existing risk control measure for a hazardous situation that could result in significant harm?
- Could the change significantly affect clinical functionality or performance specifications that are directly associated with the intended use of the device?
You can see there are quite a few important questions you'll need to be able to answer. How you document your thought process and how you ensure (in production and post-production) your products are safe and effective is up to you.