CleverNickName
Limit Break Dancing
The only "old 5E stuff" that they've confirmed One D&D to work with are 5E adventures (and "supplements," which are deliberately undefined). If the only 5th Edition D&D materials you own are the published adventures, you're right: there's little to worry about.I really have no idea why anyone cares if it is a new edition or not. If it is balanced to work with old 5e stuff, or at least enough that it doesn't matter a lot, and the basic concepts don't change, who cares? But I'm sure we'll get hundreds of posts debating it.
But I've spent a great deal of money on the 5th Edition D&D books and digital content through D&D Beyond and the Roll20 Marketplace. It's unclear how much of my current material will be compatible (or accessible) in the future, and until I know for certain, I think it matters a great deal.