I am re-scaling a document from A4 to A3 using Affinity Publisher 2.0.4. I follow the instruction from the forum elsewhere: select the master pages, change the paper size and select scaling, and then apply to all pages when the popup window comes to inform me that master page and page sizes do not fit. This mostly works fine, but I noticed three problems:
"Distance from Text" defined in the "Text Wrap Settings" are not scaled. As a result, the text is too close to some of the frames. I can fix this by manually increasing these values, but that is pretty annoying (and I am so glad that I found the setting to increase the number of decimal places for mm ... 2.82 and 2.83 sometimes made a difference, and it was shown as 2.8 - well done to think of that option!!)
I have two horizontal 2pt lines on one master ... they are scaled up to 45 pts. Quite a shock the first time I saw that That's luckily an easy (though annoying) fix.
The margins for column guides attached to the master page are scaled wrongly as well. I had 15mm distance from all sides, after scaling this was 35mm (iirc - some obviously wrong number). Again, I could manually fix these settings on the master page.