Should I move the existing point and add info to it? This will include change to postbox type (pillar to lamp) and keep the history of the old point.
Should I make a new point, with the current reference number and then delete the old point?
In both cases, should I add any info anywhere about why the old information is no longer correct?
What I would ask myself is whether I consider it the “same” post box by some metric (and it’s probably a bit subjective which criteria one would use here). If it’s the “same” one, I would move the node, otherwise I would delete it and create a new one.
Ultimately, I think both options are defensible. It likely won’t make a huge difference.
In the UK most postboxes will have a reference of the form CHxx yyy[D]. If the new post box has a different reference and/or style (pillar, lamp, wall) then I would create a new node, and change the old one to demolished:amenity=post_box. If life is too short for this kind of thing delete the old & add the new (evidence base that they are the same is pretty hard to establish, so retaining history may be misleading).
I do know of one post box where the location changed significantly but the reference remained the same (actual post box is entirely new). This is one in Armagh City which is now located outside Sainsbury’s, but caused a puzzle when I surveyed.
A pillar box damaged in the Arndale Centre bombing in Manchester was re-located subsequently, but I have no idea if the reference was kept.