First; It most certainly possible to upgrade from 2 x 2TB to 2 x 8TB.
Theoretically, you can do it without losing any data. (I would have a backup. . .although I have read that this works just fine. . . .I am not a trusting person).
Second, WD has “confused” the Red Product line. As of today;
- Red = SMR Technology, 5400 rpm drives <----- Avoid at all costs. Totally unsuitable for NAS (or really any) usage
- Red Plus = CMR technology, 5400 RPM <----- Last I heard, these were the default drives in WD NAS units. Perfectly fine for nearly all applications. Fine for streaming. Not fine for games. . .but if you are a gamer, you aren’t running the game from a NAS to start with.
- Red Pro = CMR technology, 7200 RPM <----- As good as it gets with HDDs.
Note that most manufacturers make quality NAS drives. Almost all will work in a WD NAS.
In my mind. . .the key is to ensure you get a CMR, not SMR drive. Manufacturers deliberately try to hide if a drive is SMR. SMR saves ~10% manufacturing cost, does not affect the price you pay, and really compromises performance.
7200rpm won’t be SMR. . . so you should be good to go.