Posted as a reply in the test discourse forum. Thought it might be useful to keep a record here as well.
I don't know how to do it in phpBB's (SQL?) Database.
If the data is stored in plain text, then we could just do a sed operation:
I tested it, it works. Although I doubt the data is in plain text, so we might need some SQL WHERE table=xxx contains "[youtubebb3]" voodoo magic.