Restoring old Whatsapp backup but preserving new data

Some time ago, I transferred data from an old to a new iPhone using iCloud. Recently, I noticed that something went wrong with the Whatsapp transfer. Although I backed up the Whatsapp on my old phone with iCloud and then used iCloud to transfer data to the new phone, the photos and videos did not transfer (but the messages did). In the chat, they are shown as white squares with question marks. I did click "include videos and photos" when I was backing up, though.

My Whatsapp tells me that I can restore the initial backup if I free up 18,97 GB on the iPhone (see yellow part in image 2). I could theoretically do this. However, a lot of new photos and messages accumulated on my Whatsapp since I changed my phone a year ago.

Therefore, my question is: If I restore the old backup with the missing photos and videos right now, will I lose all the new data that has accumulated since the last backup was created?

And, is there any way to restore the old backup AND preserve the new data on my Whatsapp?

Thank you.