Drupal 5.x to 8.x upgrade experiences / advice ?

Is it likely that a Drupal 5.x to 8.x upgrade will require major changes or even a major restructuring of an existing website, or is there an upgrade tool which will make the 5.x to 8.x transition relatively easy?

I’ve found some 6.x and 7.x to 8.x information, but no experiences starting from 5.x

I understand that this is a rather general question, but thanks for experience remarks & advice related to a 5.x to 8.x upgrade (I’m new to Drupal).

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