Not quite understanding - you're not saying DIM overwrote Legacy content are you? That shouldn't happen.
When downloading through DIM, it will overwrite any file with the exact same name, and if the name has been changed in any way, once you quit, and then open DIM again, only the newly named files will be in the folder you use to download files. DIM only sees what's "in your account", so if your account has the newly named files, that's all it's going to see, and the other files will disappear from your download folder. It's basically the same action if you download, install, and then uninstall something. It will go back into the folder where you originally downloaded to, and disappear from your Installed list in DIM.
I found this out the hard way when the Morphing Fantasy Dress for Genesis 1 was updated. The 2 addon packs were still in DSF format, which is what I need for my DS 4.0 Pro, but the dress itself was now in DUF format, and no longer usable. Luckily, I regularly backup to my external hard drive, or I would have been S.O.L., as the saying goes.
I do want to put 4.9 on another computer in order to look at it. I know with Connect you can not organize your data the old way via Windows Explorer to use in the Content Tab. what I'm unsure of is whether this is only the case if you use DAZ Connect or it's just how things are done in DS 4.9.
These are reasons, especially the organizing of data, that's turned me off. Even if I had DS 4.8, I wouldn't be upgrading to DS 4.9, even if I could ignore Cloud Available.