It depends.  If the change is strictly related to the generic disclosure information that makes up part of the statement "overlay" (the layout and generic text that all statements have, regardless of the exact member data or CU configuration), then yes, the change will be retroactive to existing eStatements already stored in CU*Spy.  That's because CU*Spy doesn't really store the actual statement image, but rather constructs the eStatement using a combination of the raw data generated by CU*BASE plus the instructions in the CU*Spy (eDOC Innovations) programming that organizes and presents the data so it can be viewed by members.  This is what allows us to make the CU*Spy eStatement both visually attractive as well as interactive, with helpful links to jump to other parts of the statement, or other details, or to use handy tools like downloads and interactive reconciliation features.

If the change has to do with the data from CU*BASE itself, such as the addition of a new data element being pulled from CU*BASE and shown on statements (Tiered Service scores, for example), the change will take effect on a certain date and going forward, but not be retroactively manufactured for old statements.
The advantage of this method is that we have complete and efficient control over generic statement layout and content without the time- and resource-consuming process of having to re-generate, re-import, and re-archive all statement data from CU*BASE, should we need to make a correction to what members see in the basic layout and disclosure content.