Wood type can change when firing... but does not always HAVE to.
Sometimes the change is due to limitations in the kiln's design (usually air handling capacity......but sometimes total refractory insulation value). Soft woods like pine, hemlock, fir and such release their heat very rapidly when compared to hard woods like oak and maple. So if a specific kiln has trouble getting to temperature at the end of the firing cycle when heat losses are starting to become really significant.... some folks switch to soft wood to do that last litle "push".
On a kiln that is being fired for natural ash deposit glazing (shizenyu in Japanese), the chemistry of the different ash from different species cause different effects. So when you switch woods, you get a bit of a "layering" effect on the surfaces developed like layering different glazes. More subtle... but there.
Some folks are lucky enough to be able to fire with a particular wood species for the entire firing. Just like in ash glazes, the different ash produces different effects.
One of my friends in Japan once tried that with a wood known as hiba, a type of cedar. He had never used that alone before. It turned out that for a given volume of cords of wood burnt in a firing of the anagama..... that species did not produce all that much ash! Less than with the other woods he typically fired with (mainly Japanese red pine). BUT......... the ash that it did produce produced a beautiful yellow shizenyu on the dark reddish clay body (think BIzen-yaki). Beautiful stuff.
I typically fire my noborigama now with mixed hardwoods. Oak and maple mainly, with a little birch. I used to fire it with all scrap sawmill pine and hemlock......... but all of the sawmills in the local area have closed due to the disapearance of the lumber industry in these parts. Hard to find that kind of scrap wood around here anymore. It ticks me off. I started out usingt wood from a mill about 1 1/2 miles from my kiln. That scrap wood source radius slowly speread out to about 30 miles over the years. Now...... I'd have to go very far afield to get that kind of scrap wood anymore.
The VERY few mills that still exist around here at all are now set up to instantly chip all the slab and edgings (using fuel) and blow that stuff into a tractor trailer bin... so that it can be sent by a big truck (consuming more fuel) to a central company's location (often a lot of miles away), then use yet more fuel energy to process it into nice consumer-friendly wood pellets, so that it can then be shipped by truck (more fuel energy applied) back to a retail store somewhere, so that it can then be bought by consumers and taken home (more fuel used by the car) and burned in a home pellet stove (that uses electricity to power the auger that feeds the stuff into the stove....more energy applied). Yeah.... totally absurd. Just burn the darn wood as wood as close to the original source as possibe!!!!!!. But they won;t save the stuff for me...... even if I offer to pay an equal price for it to what they get for the ships or even a bit more. Too much trouble for them.