It really depends on your party's composition, number, and options. CR 11 notionally is about 2-3 levels above where the characters are "meant" to be on RHOD by the time they get to the stone lion, but CR's also a horrible estimator of how hard an encounter actually is. For a straightforward, unoptimised group of 4, throwing them at a blue dragon of Tyrgarun's size and maturity a level or two early could be kinda dangerous. To some extent you have to eyeball it, judge whether the players can handle it.

For example, Varanthian as written on RHOD can be handled pretty easily by competent parties because of the stuff I identified in the handbook -- she has a breath weapon that's only useable every 10 rounds (yep), she has no real buffs available to her, and her prime mechanic for doing damage only works over 2 rounds, during which she's vulnerable to sneak attack.

A blue dragon's a different prospect. Even on the underpowered RHOD version, you have a much faster breath weapon, decent buffs, higher AC, and a Wand of Fireballs. Rebuilding according to the handbook, it becomes much deadlier: Knowledge Devotion, metabreath feats, better spells like Scintillating Scales, Blood Wind and Nerveskitter.

The second thing that springs to mind is that if you're going to have this fight, it should probably be all outside. Just forget about having a fight inside the stone lion (unless it's with the Ghostlord). Basically, a dragon's best means of surviving a confrontation with adventurers depends upon it being able to win initiative and move fast, otherwise it invariably gets clowned on the action economy. If Tyrgarun's going to fight on the wing, he should be flying round the back of the stone lion's head, ducking for cover behind stuff, etc, etc. You don't sit still and you use that 150 feet of movement as best you can -- in particular, since flying creatures have to hold half their movement in reserve to maintain level flight, flip around and land on the stone lion's back which is out of the characters' line of sight.

Switching out Varanthian for a different dragon is an option that's been tried before -- I know there's a discussion around it in the thread earlier, I'll go looking for it to see if anyone's done it before. Actually I might add some text around this subject, it comes up a lot.