Base Creature, and leveling up with a template

[Gonsen00]

Suppose there is a 4th level fighter with the half-fiend template. The base creature would be a 4th level fighter, thus the template would function with him having 4 HD.

Now let's say that the fighter leveled up to 5th level. What happens with the base creature? Is it still a 4th level fighter in regards to his template's powers, or does the base creature now become a 5th level fighter and gain all the perks of it? (Gain Unholy Blight as an SLA, CR mod becomes +2, gain +1 dmg from smite good, etc.)

I ask because while I can find that the base creature is what the creature was before the template is applied, I can't find any mention on if the base creature is dynamic and changes as the creature advances.

[Blymurkla]

Normally, templates are a GM thing only. And GMs do as the wish. There is no rule because there is no need for one.

If I had a reoccurring NPC with a template, I'd recheck the template each time I added a new class level. If it fits with the tematics of the template.