Essentially, the "Sephiroths" encountered throughout the first and beginning of the second discs were directly derivative of JENOVA. In other words, the headless body of JENOVA took on Sephiroth's form. We know that JENOVA has the power to change its own appearance...

Sephiroth: Cloud... Don't blame Tifa. The ability to change one's looks, voice, and words, is the power of Jenova. Inside of you, Jenova has merged with Tifa's memories, creating you. Out of Tifa's memory...... A boy named Cloud might've just been a part of them.
...and as a result, it's no great stretch of the imagination to understand that JENOVA could have taken on that form. Even in the Icicle Inn films, JENOVA is referred to by the pronouns "he" and "it." This also suggests, considering that JENOVA is often referred to as "Mother," that the entity has the ability to change its form. Moreover, the forms of JENOVA battled thorughout this period (Jenova∙BIRTH, Jenova∙LIFE, and Jenova∙DEATH) all bear a distorted resemblance to the headless body of JENOVA. While it could be argued that since Sephiroth supposedly carried the body of JENOVA with him, these appearances would not preclude the possibility of Sephiroth really being a CLONE who exactly mirrored the original, this is not consistent with what we know of Sephiroth Clones, and in particular the CLONES (the survivors of the Nibelheim disaster/the figures garbed in black). While free will and self-awareness seem to be wholly submerged, there is no evidence of any real physical transformation.

It's worth noting, by the way, that the lines regarding the ability of JENOVA to "change one's looks" follows this earlier remark by Sephiroth.

Sephiroth: Five years ago you were... ...constructed by Hojo, piece by piece,
right after Nibelheim was burnt. A puppet made up of vibrant Jenova cells, her
knowledge, and the power of the Mako. An incomplete Sephiroth-clone. Not even
given a number. ...That is your reality. Ha, ha, ha......
Now, while it's difficult to use these lines as direct evidence, since the actual thrust of the remark is a lie, it's notable that no physical form is mentioned. While the lines are not true as regards Cloud, a being (if one existed) made solely from Mako and the cells of JENOVA would be incomplete, as Sephiroth Clones are [i]people[i] injected with JENOVA cells. Such a being, then, would be constructed entirely from JENOVA, and would arguably be able to change its form.

Further, consider Sephiroth's lines immediately prior to the battle against Jenova∙BIRTH.

Cloud: No... not Sephiroth!

???: ......After a long sleep... ...the time...... ...time has...... come...

Barret: Look, Cloud!

Cloud: Sephiroth! You're alive!

Sephiroth: ......Who are you?

Cloud: You don't remember me!? I'm Cloud!

Sephiroth: Cloud...

Cloud: Sephiroth! What are you thinking!? What are you doing!?

Sephiroth: ...the time...... is now...
Sephiroth's final line here is critical. It seems, at least to me, to be the effective beginning of the Jenova Reunion. Both Sephiroth and the head of JENOVA, after all, are located to the far north; as a result, according to the Jenova Reunion Theory, it makes sense that the body of JENOVA and anyone afflicted with JENOVA cells and lacking the will/consciousness/self-awareness to resist them would move toward that location. If this is the case, however, then the direct action of JENOVA (appropriately taking, all things considered, the form of Sephiroth) is explained. More importantly, though, any action involving the CLONES would only really be starting at this point; therefore, the form of Sephiroth that tore up the Shinra Building would not be one of them.

Further ruminations on JENOVA can be found here.