What are the chances of getting into a PHD program even if your graduate research is completely different?
Completely possible!

You only need to argument your research as multi-field with an holistic approach, base your methods or the relation on technology and how it impacts the new field, or propose some innovative way to apply or relate computational sciences to biological systems (neural networks or Ai to learn how X works are always a good hit.)

As as example, my first career was in 'environmental improvement and diagnostics', then I went into 'informatique sciences' , specialization MBA into 'economy', then MBA into ' Computational Systems' but PHD in 'Organizations direction/Management'. It all comes down on how you relate everything.

As for how easy it would be to enter, well, you need to see which universities you want, go into their website's  and see their requirements for candidates. You can then tailor your proposal to them and contact the admissions department to say what they want. Even within the same country there are variants among the universities, more so between public and private ones.