Note, its up to the DM to describe the door in such a way that supports the level (and therefore the DC) given to it. So, a level 1 door might be of wood, and in really bad shape, a level 5 door is solid wood, a level 10 door is heavily reinforced wood or one with metal cladding, a level 20 door made of solid iron and a level 30 door made of admantium reinforced by the souls of the damned.
Otherwise, it may harm the players sense of immersion in the setting if at level 1 they need to roll a 15 on the die to bash down a door, and when they return to the exact same door 10 levels later, they still need a 15 on the die to bash it down. Instead, you should either keep the original DC for the door (which would make it fairly trivial to get through now), or describe in some way how the door has been "toughened up" to make it more challenging - be it through magic, or having been repalced with a door made of superior materials.
Note, in the case of doors, given that to get through one at a lower level, the players likely bashed it down, having it replaced with a new door likely makes sense. However, in other instances (such as a castle wall the PCs climbed over in the previous tier), one should either come up with a convincing reason why the DC has changed, or simply keep it at the original DC.
Personally, if there isn't some significant plot reason to have it at a higher DC, I would keep the old DC, especially if the test was quite challenging the first time - its a good way of letting the players have a "look how much more powerful we are" moment, especially if you plan to spring a "look how we aren't as powerful as we thought" moment soon after.
