As far as I know, all your content should still be available to you, you shouldn’t have to give up loading them into DS, although I can’t remember what the issue was with very old .daz files and how the upgrade will affect your ability to use them (but they should still be there anyway.). Although the question would be better asked in the DS forum, let’s see if we can DS seeing your content again.
To help, you might want to include an example of a product with full paths and screenshots where appropriate.
First thing to check, is the Runtime/Content folder mapped so DS knows where to look for it? In the Content LIbrary tab, click on the menu icon in the upper corner and select Content Directory Manager. Click on the ‘+’ to expand “DAZ Studio Formats” and/or “Poser Formats”, and make sure the path of the folder ONE LEVEL ABOVE the Runtime folder or Content folder the product is installed in is listed in the appropriate category(ies). If not, add it. For example, if you have C:\ProductX\Runtime, C:\ProductX should be mapped.
Next, make sure you told DS to search through these mapped folders for your content: In the Content LIbrary tab, click on the menu icon in the upper corner and select “Scan known directories for files”.
Then, make sure the CMS is running. (if all your custom categories are missing, it’s not running, let us know.)
Finally, make sure you are looking for the right content in the right place. There are several places it could be listed, and multiple ways to view it and/or search for it. If you give us an example we might be able to narrow it down for you.