Use Content manager search, use Location = Local and out of date = true , click the plus signs on the search box to get the options. Then Edit, select all, put mouse over highlighted assets, right click, download, that should get most of them, you may have to mop up a few in the Trainz menu though.
Some will show up as N/A (not available) in the Download Helper list as the out of date items are built-in.
Select all these items in the main list.
Right click and select Asset Versions and then click on show in main list.
Select the DS items( as opposed to the update available items which will be initially highlighted)
Copy them to the Download Helper and you now have the lot. The N/A items will still be there but ignore these and clear them at the end, as removing them while downloading seems to stop the download for me.
Finally list and delete local obsolete items (you don't need to worry about replaced built-in items).
(I have to thank the Auran team for sorting this out for me)