A theory can definitely be modified. When there is ample evidence of the theory's correctness (sorry, cannot think of the proper word there), then the theory can be declared a law.
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
these two have been modified
A scientific theory has a different definition from a "general" theory of everyday use. "Regular" theory (according to dictionary.com): contemplation or speculation.guess or conjecture. Scientific...