It looks like we are finally upgrading our (extremely) old wiki (v5.4.3) to a newer verion (v15.5.2).
Maybe upgrading is the wrong word, we are more likely to “export” and “import” pages from the old wiki to the new wiki.
Does anyone have some good advice to make this as smooth as possible?
Not “export” and “import” pages from the old wiki to the new wiki.
It’s generally much safer to upgrade the wiki in a more standard way (yes, even if it’s very old).
Our ICT have tried to upgrade directly from 5.4 to 15.5, but ran into too much trouble.
They then tried to upgrade in “steps” (from 5.4 to 7.x to 9.x, etc.) but eventually they ran into too much trouble.
So they made the decision to install a fresh XWIKI and that we must transfer data/attachments from the old wiki to the new wiki.
Does anyone have any tips on how to transfer data from 5.4 to 15.5?
See https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Backup#HUsingtheXWikiExportfeature epecially for the limitations.
Looking at those pages, am I right that XWiki 5.4.3 can only export ALL wiki pages?
Or does 5.4.3 also allows me to export one space at a time?
(I can’t find any export option at space-level, only on the wiki-level there’s “Content”–> “Export”)
Yes, 5.4.3 is too old for partial export. That being said, once you have the full XAR, all it takes to remove some spaces is to open it as a ZIP and remove the folders you don’t want.
We have around 3k pages and 6.5k attachments.
Is there a way to know the total sum in Mb of those 6.5k attachments?
If you are comfortable with SQL queries, you should be able to gather the total size of the attachments stored in the column XWA_SIZE of the table xwikiattachment.
So after a bit of trial and error (and removing corrupted attachments), I now have a 5.1Gb XAR 
I can open that XAR and see all the different spaces as folders, with the pages as XML. If I remove a folder from that XAR, do I also need to edit the “package.xml”?
No, it’s fine if package.xml have too many documents listened. It’s more of a filter than a hard list of documents that must be in the XAR.
Thanks, I have imported the very few first spaces/pages into the new Wiki.