Questions like this need good data for answers, and that data just doesn't exist. I assume you're talking about your own story, and you'll be in uncharted territory.
My guess is that it is a little of both. When people relapse after 12-18 months, they usually fall into old habits, old websites, old fetishes and so forth. That tells me that old porn tastes get reactivated, and probably the old porn circuits. Think of an old path in the woods that is grown over and barely discernable as a path... but it is still there. If you find you're living out your porn life more or less as though that sober period never existed, then it is probably a good guess that you'll be dealing with recovery stuff similar to someone who started early. It is also possible that such a relapse occurred precisely BECAUSE you started so young, and it is in deep.
That isn't to say that you didn't do yourself a lot of good by quitting for that long. That time away certainly does a lot of good, and it gives you lots of tools. It also matters how long between that long 12-18 month sobriety and trying to reboot again. A day? Week? Year? Several years? The longer it is, the more the progress erases.
Instead of worrying how bad another reboot will be, I'd just look at it optimistically. If you made it that long, "white knuckle" stuff isn't an issue for you. That stuff usually doesn't let you get past a week or two at most. What other deeper things are lingering in your life that might never have gone away? Emotional stuff? Your perspectives on sex and relationships?