I have a former customer who, contrary to my recommendations, has not stayed on top of their WP upgrades. They now need to make the leap from 3.5.1 to 4.1.1 (or whatever is current at the moment). They used a fairly vanilla theme (might have even been Twenty Eleven), and not used too many plugins.
I have found numerous threads (here and elsewhere) that talk about doing this 1 mini-upgrade at a time. Is this really necessary, or can bigger steps be taken? E.g. 3.5.1 -> 3.9, then 3.9 -> 4.0, and then 4.0 -> 4.1.1
Thoughts or experiences are appreciated.
Below are the methods you can try. The first solution is probably the best. Try others if the first one doesn’t work. Senior developers aren’t just copying/pasting – they read the methods carefully & apply them wisely to each case.
Check out the Codex on Upgrading Across Multiple versions.
Regarding the upgrading steps, it says:
If you plan on upgrading across more than two major releases, please
consider upgrading incrementally to avoid potential conflicts and
minimize the risks of database damage. For example, if you plan on
upgrading from 2.5 to 4.1.1, upgrade to 2.7 first, followed by 2.9,
then 3.1, and so on. Essentially, it’s okay to skip one release, but
never skip TWO when upgrading.
So according to this advice, your upgrade sequence could be:
3.5.1 -> 3.7.x -> 3.9.x -> 4.1.1
I agree with the comments by @Milo and @PieterGoosen; You shouldn’t experiment with this on a production install, test this first on your dev install. Deactivate plugins before the upgrades and activate them afterwards, one at a time. Remember to activate the debug log and scan through the
debug.log for errors.
Remember to backup everything.
You can find the old releases here.