Dire pet training after level 15

So out of boredom I decided to try a W/R PVP character with a pet (a real pet not an iway corpse) and was pleasantly surprised to find that pets, contrary to what you mostly hear about them, in fact do not suck. So I decide to train myself a pet on my PVE main warrior. I figured this would be a quick process as I had heard pets gain xp even when dead. This would allow me to go solo some trolls for mucho xp and get a max pet very quickly.

Before doing this I first consulted the wonderful pet guide here on guru to confirm that pets gain xp while dead, in the process of this I discover the difference between pet evolutionary states, a subject I was shamefully ignorant of. After reading up on this I decide that Dire is the way to go for me, which as any of you Beastmasters know means troll soloing is out of the question as it would result in a most decidedly hearty pet. I currently have trained a level 15 Dire Moa Bird, my question is this (and I apologize for making a short story long): once a pet reaches that final evolution does the manner in which you train it from 15 to 20, or the manner in which you use it at all after level 15 matter ever? If I decide to take a dire level 15 for some troll soloing to speed up the process to 20 do I risk knocking it back to hearty? Can I use the pet any way I see fit after level 20 and never worry about it changing states?

Feel free to flame me if this was covered in the guide, but its the only point about pet training that I came away not 100% clear on after reading it.

Once its reached its final evolution, you're free to level it up the rest of the way however you want. The isolated reports of pets re-evolving after their final stage have never been substantiated.

Thank you, you've saved me quite a bit of time.

Incidentally, while training a new (level 5) pet, I had my second Aggressive wolf (at lvl 11) turn Elder at level 16. I thought he might just remain Aggressive, which suited me fine, but alas he evolved again. So it seems you must play carefully until your pet either evolves to his final form, or until apparently, level 16.