1. "...para registrar el cambio de domicilio de la sociedad..."
= "...to register the company's change of address"
OR: "...to register the change in the company's [legal] domicile"

In U.S. law, the "address" and the "domicile" are two different things. The most important one is the "domicile," which is the physical location designated or "elected" by the company as its formal, official address, and which is the one used in legal records, litigation, commercial registrations, etc.

2) " ...modificaciones del contrato social, tales como aumentos de capital, designación de gerente y demás..."
= "...changes in the Articles of Incorporation, such as increases in capital, the appointment of a chief executive officer, and other changes..."

Jean-Jacques Rousseau notwithstanding, a company's "contrato social" is the document known in U.S. business as the Articles of Incorporation, which is the legal instrument that forms the company. It should not be confused with the bylaws ("estatutos"), which contain the specific rules governing the company's activities.
Also: In connection with the Articles of Incorporation, the phrase "increases in capital" refers to the amount of money that the company has obtained, usually through the sale of stock and/or bonds, in order to engage in itsbusiness activities. This amount is customarily specified in the Articles of Incorporation, and -- unlike the company'sincome, which, ideally, will increase on its own -- cannot be changed unless the Articles are amended.

3. "...Este consentimiento por escrito puede ser firmado en copias/duplicado."
= "...Signed photocopies or carbon copies of this written consent shall be as valid as signed original copies."

Here, a strictly literal translation ("This written consent may be signed in copies/duplicate") fails to convey the meaning in any intelligible way. To make sense in English, the sentence first needs to be rearranged ("Photocopies or carbon copies of this written consent may be signed"), and then a little understanding of business procedures has to be applied.

(Note: The single word "consent" is being used increasingly often as a synonym for "consent form" or "consent document," either of which would also serve here.)

Cheers, HC