Short answer If Sthāpanāchārya‑ji moves by mistake during Pratikraman, it is treated as a small lapse (aticār) of the steadiness vow (āsana/kāyotsarga). The Pratikraman of others does not become invalid. He should immediately admit the fault (ālochana), say “Micchāmi Dukkaḍaṃ,” regain stillness, and continue. After the Pratikraman, he may take a simple prāyaścitta (atonement) as guided by the guru/tradition.

A little more detail

• What is affected: Only his personal observance of stillness; the sthāpanā and the collective rite remain intact.
• What to do right away:

1) Mentally confess the lapse, 2) say “Micchāmi Dukkaḍaṃ,” 3) sit steady again. If the movement disturbed the sthāpanā setup, calmly re‑establish it and continue.

• After the rite: Take a small prāyaścitta according to local maryādā (for example, brief extra kāyotsarga, additional alochanā, or repeating a short samayik), as instructed by your guru or the seniormost present.

Tradition note

• Śvetāmbara: The role of Sthāpanāchārya is formal in Samvatsarī/other Pratikramans; the remedy above applies.
• Digambara: This specific title is less common in lay Pratikraman; if a leader/reader moves, it is likewise a minor lapse—do ālochana and continue.

For a simple overview of Pratikraman and its purpose, see this quick guide: