a) A complaint may be amended, prior to a determination under Section 280.410:

1. 1) to cure technical defects;
2. 2) to set forth additional facts or allegations related to the subject matter of the original complaint if, at the time of the amendment, a separate complaint could have been timely filed under Section 280.210;
3. 3) to substitute or name additional respondents;

4. 4) to add an additional issue if, at the time of the amendment, the added respondent knew, or should have known or had reason to know, of the additional issue.

   The amendment shall relate back to the original filing date.