New uses of old products can be patented. The application needs to reflect the discovery of new features of the old product and the new uses derived from the new features; or the use of a known product. Technology is used in a completely different field from the original, forming a diversion invention. In this case, it must reflect the big difference between the old and new fields. This kind of diversion is not easy to think of. To use the technology in this field, you need to make some decisions. creative labor.
I don’t know what you mean by “the core part does not need to be changed at all”. If the principle is the same but the actual implementation requires overcoming major technical difficulties, then it should be possible. If you just copy the original thing, it will be difficult to be creative.