Hello,
well, if you absolutely rely on those Alfresco Explorer customizations, then Alfresco 5 may be out of the question in the short term since Explorer has been removed in that version. Upgrading to 4.2 should be possible with some (minor) changes. Since you already packaged the old features in an AMP I reckon you aimed for relevatively loose coupling and should not depend on many internals / should not override any default screens - that would make it easier…
If by "processes" you mean jBPM workflows, then these are still supported in both 4.2 and 5 - but jBPM in general has been deprecated and may be removed in any future version.
In the long run (2+ years from now, when Alfresco 4.2 is waaay outdated), you may want to ponder re-factoring your custom UI parts and integrating them in the Share client.
Regards
Axel