What is the threshold for securing a patent for a modification…

You can patent improvements of products/methods as well as patent systems that incorporate known product as long as your proposed improvement or system is new, useful, and non-obvious in light of the existing products and systems (i.e. the "prior art"). There is no definite threshold or guideline other than this.

However, although you can patent improvements to a product, that does not give you the right to make and sell it because you still need to get a license (permission) from the owner of the underlying product (assuming it is covered by a patent)

Similarly, you can patent a system that includes a known product, but if that product is patented then you need to get a license to that product/patent in order to make & sell your system.

Patents give the right to exclude others, not the right to make & use the product....becuase you still need "freedom to operate" (i.e. getting licenses/permission from other patents that you are utilizing)

Here is some info on patent requirements:

New/Novelty Requirement

In order for an invention to be patentable, it must be new as defined in the patent law. This novelty requirement states that an invention cannot be patented if certain public disclosures of the invention have been made. The statute which explains when a public disclosure has been made (35 U.S.C. Section 102) is complicated and often requires a detailed analysis of the facts and the law. The most important rule, however, is that an invention will not normally be patentable if:

• the invention was known to the public before it was "invented" by the individual seeking patent protection;
• the invention was described in a publication more than one year prior to the filing date; or
• the invention was used publicly, or offered for sale to the public more than one year prior to the filing date.

One of the most important lessons to learn from this requirement is that there is a one year period after the first pbulic dislcosure or offer for sale of an invention during which a patent application must be filed. This "statutory bar" is unforgiving, which means that an inventor who does not file for patent protection on their new invention within this one year grace period will lose all right to obtain patent protection on the invention. In fact, it may be that by simply explaining your invention to friends and co-workers without any obligation of confidentiality, you may have started the "ticking" of this one-year clock.

Although the United States grants the one year grace period described in the last two rules above, most other countries do not grant such a period. Therefore, it is almost always preferable to file a patent application before any public disclosure of the invention. Most patent attorneys will try diligently to file a patent application prior to any public release or announcement in order to allow international patent filings.

Useful Requirement:

The patent law specifies that the subject matter must be "useful." The term "useful" in this connection refers to the condition that the subject matter has a useful purpose and also includes operativeness, that is, a machine which will not operate to perform the intended purpose would not be called useful, and therefore would not be granted a patent. In most cases, the usefulness requirement is easily met in computer and electronic technologies.

Nonobviousness Requirement:

If an invention is not exactly the same as prior products or processes (which are referred to as the "prior art"), then it is considered novel. However, in order for an invention to be patentable, it must not only be novel, but it must also be a nonobvious improvement over the prior art. This determination is made by deciding whether the invention sought to be patented would have been obvious "to one of ordinary skill in the art." In other words, the invention is compared to the prior art and a determination is made whether the differences in the new invention would have been obvious to a person having ordinary skill in the type of technology used in the invention.

As can be imagined, the determination of whether a particular change or improvement is "obvious" is one of the most difficult determinations in patent law. In order to make such a determination, an examiner in the patent office will normally review previous patents to find those patents which are closest to the invention in which a patent is sought. If all the features of the invention can be found in a single patent, the examiner will reject the patent as lacking novelty (that is, it is exactly the same as what was previously known and therefore is not new). If no patent contains all of the features, the examiner will attempt to combine two or more prior patents, and attempt to find all of the features in a combination of those prior patents. If the examiner is successful in finding such a combination, the examiner will generally reject the invention as an obvious combination of items known in the prior art. However, there must be some reason to combine the two references, and often a rejection based on such a combination can be overcome.

Some changes to known products which would not normally be patentable are:

• the substitution of one material for another, or
• changes in size.