IB is a two year programme and the cost of changes made in the middle varies enormously with the kind of change.
Changing schools
A move between two IB schools is possible and happens regularly. The new school takes the student into its own registration and the process continues.
The real issue is whether the subjects match: if the new school does not offer the same subjects at the same levels, the student has to change a subject and a year of that subject is lost.
So before a transfer decision, the new school's subject list and levels are compared.
Internal assessments
The student's internal assessment work belongs to them and can continue at the new school. But the process runs through the school: the new school's teacher takes on supervision and feedback.
Continuing a half finished piece with a new supervisor costs time and sometimes means starting again.
The extended essay and the core
The extended essay, theory of knowledge and CAS also move. CAS records are the student's own and have to be transferred into the new school's system.
That transfer is administrative and, when skipped, the core appears incomplete at the end of the year.
Joining the programme midway
Spending the first year in another system and joining IB in the second year is not realistically possible. The internal assessments and the core began in the first year and cannot be completed retroactively.
In that situation the options are starting IB from the beginning or continuing in another system.
Timing
The least damaging moment for a transfer is between the end of the first year and the start of the second. A transfer in the middle of the second year puts both the exam registration and the internal assessment calendar at risk.