Prospective or retrospective effect of amendments affects whether clarificatory, substantive, curative, or evidentiary changes relate back or operate forward. Determination whether an amendment is prospective or retrospective depends on examining the statute's scheme before and after amendment; clarificatory amendments are treated as retrospective as declarations of long standing legislative intent, substantive amendments altering rights require clear wording to be retrospective, curative amendments may relate back to remedied provisions, and amendments prescribing evidentiary or uniform valuation methods may be given retrospective effect to secure uniformity.