I'm interested to know how it can change ? Are you a shapeshifter or simply metamorphosing into something else ? 
Most of the ancestry companies (Ancestry, 23andme, etc.) calculate ethnicity by comparing your DNA with their huge databases of DNA reference samples from around the world. There are several issues that affect the accuracy - Many European countries make it difficult to have your DNA testing, so they often have to go with people who claim that their ancestors came from X country.
Also, European history has been pretty incestuous, with borders changing every few decades or centuries. For instance, it's very, very difficult to pick out French ancestry from the general West European background.
Finally, the calculate a person's ancestry using software models (principal component analysis, among others) and as the software and reference samples grow, the ethnicity estimates will change.
In general, the changes will be in the 1%-8% range. I recently have had a "large" increase in Basque (from 1% to 6%)