Is It Listed for Use?
The question is really a "Listed for Use" question.
Everything in a fire alarm system has to be "Listed for Use". For instance, a fire alarm control panel that's also listed to be used to release fire suppression gas, can be used to release fire suppression gas. Whereas the term "Listed for Use" is a legal thing, "Listed for Use" is actually based on an electronic function.
The "List" that is being referred to is a list of things that has been tested and found, not only to be safe, but to perform the function its listing. If something has not been tested and found to work, it is not on the "Listed for Use" list.
The National Fire Protection Association, Inc. (NFPA) does not maintain any list of equipment, they don't test any equipment, they don't even certify any fire alarm system. Check the definition for the word "Accepted" in the Appendix located in the back of the NFPA 72 Code book.
The "Lists" are kept and maintained by third-party, nationally-known testing laboratories like UL, ULC, FM, CE, CCC, etc. When they perform the tests, they include exactly what is used as far as tested equipment, and they also include interconnecting wire size, wire type (shielded or not shielded), and length of wire used in their test.
When it's a fire alarm system, once the test shows the system works, the testing laboratory includes all of it, including any wire they used in their test, on their list as a package.
The installation manual for the system and the installation sheets that come with the parts of the equipment show how to install the system. Looking at the manuals and sheets from a different direction, they show how the system was tested by the testing laboratory.
As long as the installation manual and the installation sheets are followed, exactly, the finished installation should work as advertised.
If the installation manual and the installation sheets show that the speaker distribution system can be used as a 25-volt system or a 70-volt system, and the wiring was installed according to the manual and sheets, the system can be used either as a 25-volt or a 70-volt system. Just make sure to follow the installation manual and installation sheets.
The installation manual and installation sheets are often available on the web. If they aren't, check with the technical support team for the manufacturer. They paid for the listing; they will know what can be done.
Power Limited versus Power Un-Limited
There is a clear delineation between systems that are power limited and power un-limited. Commonly, this is referred to as low voltage and high voltage.
Another way of looking at the difference is that anything power limited does not have enough voltage to electrocute anyone, and does not have enough current to start a fire. Power un-limited, on the other hand, has enough voltage to electrocute, and/or enough current to start fires. This is a safety issue.
Fire alarm systems installed nowadays are generally power limited. If the speaker wiring system was installed as a power limited 25-volt system, by upgrading voltage to 70 volts, the speaker wiring system will become a power limited 70-volt system. In essence, they both fall under the Power Limited category.
As far as the wiring is concerned, outside of the voltage/current ratio, there is no difference between a 25-volt system and a 70-volt system.