The trial court has no discretion to refuse a trial amendment unless: (1) the opposing party presents evidence of surprise or prejudice; (2) the amendment asserts a new cause of action or defense and, therefore, is prejudicial on its face and the opposing party objects to it. See Chapin & Chapin, Inc. v. Texas Sand & Gravel Co., 844 S.W.2d 664, 665 (Tex. 1992); Hardin v. Hardin, 597 S.W.2d 347, 349 (Tex. 1980); Lege v. Jones, 919 S.W.2d 870, 875 (Tex. App.-Houston [14th Dist.] 1996, no writ). An amendment is mandatory if it is merely procedural in nature such as conforming the pleadings to the evidence at trial. See Chapin & Chapin, Inc., 844 S.W.2d at 665. An amendment is not mandatory if it is substantive, i.e., changing the nature of the trial. See id. If the amendment is not mandatory, the decision to allow or deny the amendment is within the sound discretion of the trial court. See State Bar of Texas v. Kilpatrick, 874 S.W.2d 656, 658 (Tex. 1994). Therefore, the trial court's decision to allow or deny the amendment may be reversed only if it is a clear abuse of discretion. See id. A proposed trial amendment, which asserts a new cause of action may be prejudicial on its face. See Whole Foods Market Southwest, L.P. v. Tijerina, 979 S.W.2d 768, 776 (Tex. App.-Houston [14th Dist.] 1998, pet. denied). Merely because an amended pleading asserts a new cause of action, however, does not make it prejudicial to the opposing party as a matter of law. See id. Rather, the amendment must be evaluated in the context of the entire case. See id. An amendment prejudicial on its face has three defining characteristics ascertainable from the amendment viewed in the context of the record. First, the amendment must assert a new substantive matter that reshapes the nature of the trial itself. See id. Second, the new matter asserted must be of such a nature that the opposing party could not have anticipated it in light of the development of the case up to the time the amendment was requested. See id. "Merely because the opposing party did not anticipate the issues in the amendment is not the test. The question is whether the opposing party could have anticipated the newly asserted matter as revealed by the record of the case." See id. (quoting Smith Detective Agency & Nightwatch Serv., Inc. v. Stanley Smith Sec., Inc., 938 S.W.2d 743, 749 (Tex. App.-Dallas 1996, writ denied)). Third, the opposing party's presentation of the case would be detrimentally affected by the filing of the amendment. See id.