Ask HN: Ecommerce frontend and backend migrations

I work with an agency and in the past couple of years we've taken on several ecommerce projects that involve either rebuilding the frontend and/or migrating the backend to a new platform/CMS/database etc...

Each project has gone far beyond the initial scope of work because of conflicts with the legacy product data, and I'm curious as to how other developers have handled this issue in the past? It doesn't seem like it's a localized type of project, because every agency I've been with has dealt with similar projects.

Based on our experience so far, I'm inclined to suggest that from here on we say "No" to any ecommerce project that has an existing product database. Either that or contractually require that no work be done on any redesign until all product data and assets are migrated to the new platform, which then becomes the sole source of truth for the data. Or just triple/quadruple the estimated time frames and budgets.

I recognize these are knee-jerk reactions, but I'm legitimately curious how other developers have handled this and would greatly appreciate some constructive perspectives.