The 5e CR are more non-binding suggestions, and the calculation formula in the DMG doesn't match up with the CRs from the MM on many occasions.
That said, if you want to up the challenge, I recommend to either increase the damage output or increase HPs. Increasing AC can be tricky, because it doesn't scale as smoothly, and even a single extra point or two can make quite the difference. The same goes for handing out extra attacks.
Since we are talking about a dragon, you could also take a different dragon that has a CR that's higher by one point, and colour-swap it.
HP and damage output make more difference than anything else, or so more mechanical minds than mine have said. I like to give monsters an extra ability or two, to spice things up and keep the PCs on their toes.
Even before I really got the hang of CR, I stopped paying attention to it. Between some experience paying attention to CRs and getting to know the capabilities of my players, I just found CRs to not be very useful.
So, once you get a basic sense of the scale of things, don't bother with CRs, except maybe occasionally to sort of check in and make sure the PCs are up against the kinds of challenges you want.