Easy bit first; your new data will merge seamlessly with the Nexus data - no worries there at all.
Then the connection - I wouldn’t worry unduly, the USB runs pretty well for the great majority of users.
You may need to update to the latest driver, you may need to add ferrite chokes to the USB lead and AC power cord - but it might work fine out of the box.
You probably need to get everything running in the weatherlink software first even if you don’t intend usung it, then use the same settings for Weather-Display.
This is important:
When setting it up in weatherlink it needs to run as a Virtual Com port or it will not work in WD
This is simply a matter of not checking ‘USB’ at the crucial moment.
It does say this quite clearly as an instruction while you are doing it but the signicance of getting it wrong does not become obvious until later!
You can reset it with a small download from Davis website if need be.
Like most things the changeover is easy ‘when you have done it’ !
There are pitfalls but nothing that can’t be fixed after some confusing moments.