You should be able to just provision the 40G ports, get link up and verify connectivity and then decommision your 10G VCP links.
EX4300 supports dual-rings where eg. EX3400 is single-ring, where you had to remove a single 10G link at a time a reprovision as 40G.
If I remember correctly, dual VCP links are not weighted so both 40G and 10G will receive same amount of traffic... but as you are going from 10G to 40G that's not a problem 🙂