Greg's preliminary diff of 1.8.5 against 1.9.2 shows that there are many, many differences. The database itself is also different, according to a response to my query on the B2Evo site. I've posted another query for documentation of the differences; I couldn't find any.

What we're learning now suggests that the best approach is this:

1. Copy the old db to a safe location.
2. Do a new install of 1.9.2 in a VM, pointing it at the copy of the old db.
3. Examine carefully what is imported into the new db:
   • What fields have changed?
   • Was our customization carried over? This is the info about the customization:
     The evo_posts table contains the fields which store individual posts. An integer field has been added to that table:
     post_minutes_worked int(11) UNSIGNED No 0
4. Do any changes in the db field affect our customization code? In other words, have any fields which are referred to in our custom code changed name, disappeared, or moved?
5. Are there any other changes which affect our customizations? (For instance, does our HCMC skin work.)

Once we have this information, we can assess the practicality and desirability of moving to 1.9.2. If we do move, we'll need to approach it like this:

1. Bring down the existing blog system.
2. Archive it carefully.
3. Build the new system, importing data from the old.
4. Re-integrate each of our customizations, testing as we go.
5. Once all is working, bring the new system up, on the URL of the old system.

Since everyone relies on the blog for daily work, this might be a job for a weekend.