Trying to go schemaless

In my post SQL or NoSQL two days ago, I was dancing around the problem which is I am trying to avoid using a schema for the game’s database tables. NoSQL allows this, whereas a relational database of fixed schema tables (SQL) s more restrictive.
One reason for this is to make it easy to add new features without having to run an upgrade every time I want to add say a column in a table. I’m moving towards some key of key-value store. This is not a real-time game but a more traditional data-processing engine which reads in a lot of data then process it and then writes out the altered data.
The data quantities are not really that big, a maximum of 10,000 players in one game using 10 KB each so maybe 100 MB of data so it could all be managed say by storing the data in JSON files rather than a database.
In the past I might have used a SQL database and I have used SQLite in a desktop application. It is fast, but if I can hold everything in RAM it will be faster still. JSON does bulk up data a bit but nothing like as bad as XML which can be five-six times bigger.
Back when I programmed something comparable to this in 1989-1990, these (in the picture) were the standard media. Hard disks had not been around long and were quite expensive! These could hold a whopping 1.44 MB each.. CDs for data did n’t even exist then.