It's not hard, no, but it usually doesn't happen in the average application. That's the issue: it's not built in, it's not standardized, and every SQL database

It's not hard, no, but it usually doesn't happen in the average application. That's the issue: it's not built in, it's not standardized, and every SQL database is fully mutabile by default. If your system operates in this journaled/event-sourcing way at the most basic level then you have the ultimate future-proof storage layer. You could decide to completely change the way the data is stored and represented (in-memory or otherwise) at any time, as long as you have that raw history.