The upgrade exam is for them who already have older certification. So, in my opinion,
you should go for OCAJP + OCPJP 7.
Also, there is no dedicated book available that treats only OCPJP 7 upgrade exam preparation (apart from
this one; however, I am not sure about the depth in which the upgrade exam topics are covered).
The exam code for upgrade exam is 1Z0-805.