In order to change R&D paths you MUST be the 1st driver. Equal status will not allow you to do it.
To be the first driver you need to beat your teammate at every item on the "teammate challenge". You will get reviewed 2 times - At the end/beggining of each season and at race 10 - Germany. You will need to go from 2nd driver to equal and then to 1st. So it may take a whole season. If you Change teams make sure you will get Equal or 1st driver, or you will need to go an entire season to get 1st driver and be able to do this.
When you are first driver the team will ask you what path you want to take. Change it and after the next Race with and R&D objective (the little gear icon on top of the race name) you should get it.