One meeting has to move — does the whole pattern change?

No. A session that has already been created can be moved to another time or date on its own without touching the class's schedule pattern, so the following weeks stay where they are. Teacher clashes and capacity are re-checked at its new position.

The reverse holds as well: editing a pattern does not delete sessions whose attendance is already filled in. Patterns and sessions are stored separately — that is what makes a mid-semester schedule change safe.