Astrid, you lose access to any on-line assets.

You can download the Online Content Catalogue components before the expiry date.
Open the Local designs Gallery.
There are 7 Blue folders. Right-click on each one's title and pick Download All.
If you have XDPXv18.5, you could be downloading and storing about 3GB of data; it won't be fast.

If you have a second machine, install the application and do the same there.
This gives you a spare as if you have to re-install the application, it rolls back to be purchase version and you lose all of that year's updates.

Provided you have your design XAR/WEB files available, you will be able to edit and republish.
I have sites done in v15 and I have updated them in that version over several years without any problem.
You benefit from a later version with their improved HTML Filter, the thing that generates the website.

Acorn