This is an interesting question about which I have read or heard very little. Hopefully, someone is knowledgeable enough to shed specific light on this.
One item I can relate is that Zhang Yun in his book, the Art of Chinese Swordsmanship, states that Taiji sword is of fairly recent origin and that a specific form was not created until the generation of Yang Luchan’s sons and students.
For whatever its worth, I note that Zhang's form is quite different from Yang Cheng Fu's. Zhang's form apparently comes from the Wu Taiji lineage ("Northern Wu"?) that stems from Yang Luchan, Yang Ban Hou, and Quan You, but not his son, Wu Jianquan or the Hao family.
Whether or not Zhang's statement is accurate or generally accepted, it calls attention to the fact that many discussions of form origins occasionally give the probably incorrect impression that the emphasis on and relationship between Taiji forms, principles, exercises, and drills has remained constant over the last two centuries. Which of these forms of expression is viewed as primary and in what circumstances has likely changed.
From what I have read, it is apparently not clear that in Taiji’s earlier days, changes in form practice or in particular postures implied differences in style or had much significance by themselves. This seems to have remained much the case, for instance, with spear practice. As a result, Yang family sword practices, drills, and principles may have predated the establishment of consistent forms by subsequent lineages of Yang style.
Also, as you may be aware, some might dispute a simple statement that Yang hand forms are derived from the Chen forms. There is of course complete agreement that the Yang family learned Taiji from the Chen family; however, the precise relationship between current Yang forms and current Chen forms is subject to several levels of disagreement. This lack of agreement precludes consensus around viewing any extant Chen form as being a simple linear ancestor of any extant Yang form. I do not say this to raise controversy, but simply to acknowledge the existence of layers of possible discussion. In my opinion, some people seem too ready to reduce historical facts of the relationship between Yang and Chen family practices to a description of mere variation, refinement, degeneration, or simplification of practice. Life is probably not that simple.
Good luck with your search,