smarty notices in manager after upgrade to 2.6.1 (undefined index: style)

I'm testing an upgrade from 2.3.3 to 2.6.1 (upgraded January 2018). The site is up-and-running for the most part.

One of the problems I'm trying to solve is php error notices being reported when editing template variables in manager.

Here's an example:

Notice: Undefined index: style in /.../cache/mgr/smarty/07c5511eef8fe647877eb0296b627917a870236f_0.file.textbox.tpl.php on line 39

I've seen some arcane discussion of what seems to be a related issue here: https://github.com/modxcms/revolution/pull/12889/files.

Any ideas what I might do? I mean I guess I'll simply be turning notices off at some point, but is it wise for me just to ignore this?

If you can revert to 2.3.3, you should upgrade to 2.4.0, 2.5.0, 2.6.0, then 2.6.1. There are some changes to the DB structure and files that occur only in the major version upgrades.

Ah, that's really useful information, thank you, but oh man it's bad news...

Ok supplementary question, if you don't mind...

How substantial are the database changes? Would it be a feasible amount of work to start with a fresh install of 2.6.1 and write some sql to populate the 2.6.1 database structure with the data from the old 2.3.3 database? Have you heard of anyone doing something like this?

It's an important question in another respect because I can't take the site offline while carrying out what will be quite a long and risky upgrade process. This means the production site (that will still be on version 2.3.3) will change after I upgrade a copy. I had hoped to simply update the upgraded database with the latest changes in the production database and copy over any content contributor's files just before I was ready to do a quick swap of the upgraded version into the production position.

I saw that the database of 2.6.1 was different to 2.3.3 on a fresh install but after doing the upgrade and seeing no database changes I had assumed there must be some, er, backward compatibility!