When you say "CML advancing" you are discussing CML progressing from Chronic Phase to Accelerated Phase and then Blast Phase. Drug resistance does not fall into this category in most cases, since another drug will normally work to keep the patient in Chronic Phase.
For CML to advance, it will first morph in some way that makes it more aggressive. In that way it begins to act like an acute leukemia, moving quickly and not responding very well to TKI drug treatment. If this is going to happen, it will likely happen during the first couple years after diagnosis, with the higher probability during the first year. Even then the statistics are quite low for disease advancement.
It is a controversial issue whether low blood counts along with increasing blast count alone represents an advancement in disease phase. Oncs assume that it does. Although the term "blasts" in leukemia is assumed to always be bad, it is not. There are good blasts and leukemic blasts. Good blasts are a sign that the body is trying to fix the low blood count problem by making lots of new cells, and blasts are a stage in that blood making process. This would just be a somewhat short spike in blast count. But increased leukemic blasts are a possible sign of disease acceleration, although traditionally Oncs would also look for other signs such as increased basophil counts and so forth. So blasts are only a problem when they are leukemic, but Oncs do not take the time to figure out if they are or not. That is a bad practice. If blasts are increasing but FISH and/or PCR from the marrow are not, then I would not call that a problem. It is an issue that needs more research to define which is which.