Going I-5 to I-80 is about the worst choice you could make for a trip like this - unless you've got a very specific reason.

First, you're increasing your travel distance by almost 50% - going from a 1300 mile trip to one that's nearly 2000 miles. That means you need to be on the road for at least one and a half more days, which is one and a half more days you need to be on the road, and one and a half days more where you could see winter weather.

Second, you'd actually be increasing the number of mountains you're dealing with, as you'll have to both deal with the mountains in Southern Oregon/Northern California as well as Donner Pass over the Sierras.

Its impossible to know what the conditions will be on the road until you have a specific weather forecast, but if there is bad weather predicted for I-84, then the alternate route I'd be looking at would be I-90 to I-25 through Montana. Frankly, even if there is bad weather forecasted on both of those routes, I'd choose to simply wait an extra day to travel, rather than go all the way down to California.