Do I have to request an altitude change (CENTRE) if I have been cleared to follow my flight plan?
No, you don’t have to. However, if ATC assigns you a altitude (should there be a conflict with another aircraft), you must follow ATC instructions.
And when I change from CENTRE to Tower do I have to re-ask for runway landing?
And when do I change from CENTRE to TOWER?
When they give you a frequency change. You could also be assigned to Approach.
Do they give me frequency change or do I have to ask?
Yes. You need to call inbound for landing.
Please refer to the Infinite Flight User Guide which I’ve linked below. It contains all the necessary information you need to know.
They will give you a frequency change.