What is the test for a plaintiff to amend their complaint?

"The plaintiff bears the burden of proving there is a reasonable possibility of amendment. . . . [] To satisfy that burden on appeal, a plaintiff 'must show in what manner he can amend his complaint and how that amendment will change the legal effect of his pleading.' . . . The plaintiff must clearly and specifically set forth the 'applicable substantive law' . . . and the legal basis for amendment, i.e., the elements of the cause of action and authority for it. Further, the plaintiff must set forth factual allegations that sufficiently state all required elements of that cause of action. . . . Allegations must be factual and specific, not vague or conclusionary. . . . [] The burden of showing that a reasonable possibility exists that amendment can cure the defects remains with the plaintiff; neither the trial court nor this court will rewrite a complaint. . . . Where the appellant offers no allegations to support the possibility of amendment and no legal authority showing the viability of new causes of action, there is no basis for finding the trial court abused its discretion when it sustained the demurrer without leave to amend. . . ." (Rakestraw v. California Physicians' Service (2000) 81 Cal.App.4th 39, 43-44, citations omitted.)