In Laravel's Defence (words I never though I'd say) the L3 -> L4 transistion was a nightmare because he went to PSR4/rewrote a bunch of stuff, I got bit by it as well.

All subsequent upgrades have been easier (if not always easy) by comparison - it was kind of a one time thing.

4.2 -> 5 is a problem. Pretty sure official docs recommend creating a new application and porting it rather than trying to perform a 4.2 -> 5 upgrade.