Upgrading of TiddlyWiki Classic is known to fail with some savers. This plugin is meant to fix this for some cases: at least for Timimi or MTS 1.7.0 and above.
It also adds optional upgrade autocheck on start; tiddlers and fields sorting so that the changes are easier to review.
It is tested at least on upgrading 2.6.5, 2.9.2, 2.9.3 → 2.9.3, 2.9.4. Presumably, once it covers more savers, it will be merged into the core.
Available to install via ExtensionsExplorerPlugin, like many others (to find it, install the central collection and YLExtensionsCollection from it).