It was not clear to me how to advance to WRC 2 level in career mode, until it actually happened, so I wanted to help others who may be feeling trapped in the WRC Junior class.
After 2 full seasons of WRC Junior (and winning both years), I still wasn't able to get a contract offer in the next class up. I had thought maybe I needed to level up further, but it turns out what I needed was a 'Manufacturer Tryout' event to pop for me. I am not certain, but I believe the way to increase the odds of getting this event is to build your relationship with a single manufacturer as high as you can, through selecting and satisfying the 'Manufacturer Race' options on your days between Rally events. Do this enough, and eventually you'll get a tryout offer, and from there you just need to beat the stage time and you'll be offered a new contract in a higher class on the spot. Once you accept new contract, the achievement should pop.