Hello @Heike33065578anyp
This is a complicated problem. I will try to describe a solution in steps. First, I do recommend making a full backup of your device until you are satisfied you have all images stored inside your new Lightroom account and also on separate backup storage.
The first option I would recommend you try: create a shared album in your new account, and then invite your older account to join the same album. Once joined as a Can Contribute or Can Edit member, your older account can copy all its images and videos to the album of the new account. This will copy images that exist in the 20 GB cloud storage plus any images in your old account that exist only on your device. The best thing about this method is that your previous edits will be saved and copied to the new account! See this page for info on how to create a shared album in your new account, and invite your other older account: https://helpx.adobe.com/lightroom-cc/using/save-share-photos.html
Another file recovery option available to you:
Your first account that has 20 GB of Lightroom cloud storage: It is possible that after filling the account's 20 GB of cloud storage, the Lightroom did continue to import images. Some image that could not sync may be available only on local device storage inside the Lightroom app. If you have a computer, I recommend using iTunes or Finder File Sharing to copy original images from the Lightroom application cache to your computer.
For older versions of Mac OS that still have iTunes, or on a Windows computer with iTunes, you will follow these Help steps using iTunes: https://helpx.adobe.com/lightroom-classic/kb/recover-lightroom-mobile-photos-itunes-file-sharing.html
On newer Mac OS versions, this iOS Files browsing function moved into Mac OS Finder: https://support.apple.com/en-us/HT210598
From inside each account folder with a very long name like ABCD1234ABCD1234ABCD1234ABCD1234, you will want to copy the folders named Original (your original photos without edits) and userStyles (your personally created presets) to your computer.
You can then import all these files into your newer, larger account by dragging and dropping images into the Lightroom web app at https://lightroom.adobe.com Unfortunately, there is no easy way to re-import any custom/personal presets you may have without having a subscription to Lightroom Desktop for Mac/Windows.
I know this is a lot of info. Please take your time, and follow the steps on the help pages shared above.
Wishing you all the best!
--Charlie