SQLite doesn't have the ALTER CHANGE statement (or any other way to mutate a column type)
What do we have as solutions?
1. Suggest a manual update
Provide manual instructions to site administrators, so they can choose the best method suitable.
In some case for example, we can rewrite the SQL schema: http://stackoverflow.com/a/6684034/1930997 (but not sure for the blob type)
2. Use a temporary table
Jaime suggests if we really want to support that, as the worst case scenario to use a temporary table: CREATE; INSERT...SELECT; DROP; RENAME;
3. Make a statement the extension doesn't support SQLite.
The extension is intended to environments with multiple users to discuss together
Such environments tend to use MySQL, not SQLite.
Unfortunately, we will not be able to support SQLite for installation of Structured Discussions, since that is a relatively infrequent use case. We are going to document on Mediawiki.org that SQLite is not supported.