You'd have to call Graco to get permission to use the old assembly on the new harness.
My understanding and observation is the material the straps are made of is different in texture, fibre and thickness. There was another thread about it awhile back, forget who commented on it, I agree with them, you shouldn't be swapping them.
As for the new/old buckles. Unless you have an escape artist you are trying to keep buckled, I am not a fan of the new buckles. My 4yo could do the SJKP, Britax and old Graco style buckles no problem. He then and now (almost 7yo) can not do or undo the New style Graco or Evenflo ones. It just makes work for me which doesn't impress me.
Should you need to, you may be able to request the old style buckles/harness, I got them not that long ago...
tam