The problem isn't necessarily that their style is changing. The problem is that their style changing is in effect reducing the quality of Bodom's material.
I totally agree with your point that bands need to change in order to stay interesting, as you can't expect the guys to be into the same music and influences for 7 years. But they simply arent making the transition well.
Death changed their style with Individual Thought Patterns and Symbolic and made that change flawlessly. The quality of the music was the same as the older material (in my opinion better) but brought in different influences.
Bodom are taking a step back with their music, they are losing the sound that got them famous and simply not putting out material up to their standard. If you ask the majority of Bodom fans they will mostly say they enjoy the older stuff more, and there is a reason for that.