Significant corrections Case Mix

[BRSAMI]

I recently started a new position as MDS Coordinator.  Case Mix State.    While completing the current MDS I have found that several of the prior assessment were coded inaccurately and we got lower RUG levels.  Can I do significant corrections of prior assessments to get an accurate RUG level for the patient?  I find that major information was not coded so how can the care plan be correct?  (diseases, conditions and true adl levels) Any guidance will be appreciated.