My bad. These are not actual CHANGES, these are comments!
I guess you can Print with Content Summary these as a PDF, using Adobe PDF Printer, from the Options menu of the Comments panel in Acrobat.

Comments is not considered as art, it’s just your client used them as a tool to convey the changes :)
Does it solve the problem for you?

Ah, this PDF was NOT produced by Illustrator in the first place, it’s made with InDesign, according to the code! Explains some things, but not others.

Rupert, this is strange, but for a different reason...
When a document 'saved' as a PDF from Illustrator, it gets two parts in it: a PDF part any app that understands PDFs can view (and edit, in the case of Acrobat and other editors), and the second part, in PGF format, the actual native format of Illustrator files. These look the same, but the PDF part is a basically exported, converted, expanded and flattened copy, as much as the chosen standard allows.
When Ai handles the 'saving', it just sync the PDF part to match the PGF layer... But Acrobat cares not, since it can edit only the PDF part.
But when changes are made by the external app into the PDF... Ai usually gives a message when a file like this is read back, "This file has been modified outside of Illustrator. Do you want to continue?" — but not for this file for some reason... Do you see one?

Illustrator values the original native data over the external changes, and there is no easy way to make only some data to be swapped from the PDF into PGF — these changes can be anything, not only a text change... There is a way to force Ai to completely ignore the native PGF block, by removing it in Acrobat (Save as other, Optimized PDF, and uncheck all options but the 'Discard private data of other applications' in the Discard User Data section of the PDF Optimizer dialog).
But this, or writing a PDF without the 'Preserve Illustrator Editing Capabilities' option checked with your client will ruin the editability and the original structure of the .ai file read back, making it the same as importing a PDF, without backwards compatibility...

I definitely understand the problem myself, but I don’t see a simple solution to this.
More to it, I don’t understand why the changes made by your client into the PDF are not rendered when I place it as a link into a new document! And I can’t bake these changes with the Optimize method above — the year is still visible in the imported document, with no rectangle on top of it.
That is strange. Thanks for sharing this along with the file.

Here is the file that opens differently in illustrator than in Acrobat, please if you can figure out this programming error this will save a lot of time and money because now reprints have to happen for a program error.