In general, anything that lowers your guardrail score is a risk during an upgrade. If your score is 100%, then you should be able to drop in a new version and things should just work. There are caveats around functionality which gets deprecated and eventually, down the line desupported, where you may need to rework things, but after the upgrade the guardrail score should change to reflect that. If you are running with a low guardrail score today, your system may work fine, and may be built exactly to your requirements, but going from one major version of the product to another is going to require at the very least thorough regression testing to make sure that none of that custom/non-guardrails functionality stops meeting your needs.
I'm not aware of a specific article that calls this out, but I'm sure any documentation around the guardrail score should make mention of things like this.