Document-oriented databases, with their flexible schemas, provide a nice solution. We can have older documents which vary a bit from the newer ones in the archive. The lack of homogeneity over time may mean that querying the archive is a little harder. However, keeping the data is potentially much easier.
I think this is pushing the schema migration issue from data to code, which might actually be a good idea.
This blog is called myNoSQL and it is written by me, Alex Popescu, a software architect with a passion for open source and communities.
It records my readings, learnings, and opinions on NoSQL databases, polyglot persistence, and distributed systems -- subjects that I'm passionate about.
The opinions expressed here are my own, and no other party necessarily agrees with them.
If you feel I'm biased, I probably am.