IB to IB: a seamless transition?
The transferability of the two-year IB Diploma Programme mid-way depends largely on each school's policy. You have first to match up the courses as well as possible, and then it depends to some extent on the content and sequence of teaching in the courses. The IB courses offer a lot of flexibility in terms of content within the requirements (eg which novels one reads in the Language As) and the sequence of the teaching of topics (such as in the experimental sciences) and so it means that the receiving school has to look at the syllabus of the sending school and figure out how much catch-up/repetition would be required.
The many UK schools now doing the IBDP may choose to steer away from enrolling pupils halfway through (essentially three-fifths of the curriculum must have been covered after the first year because IB exams are in May, so the last term is mostly review and exam-taking), because of the difficulty for school and student bringing the student up to speed in such a fast-paced programme, and because the school's results do appear in the League Tables.
It may be that an international school offering boarding would be more flexible and let the student join the second year (say ACS, TASIS, St. Clare's or even Anglo-European which is a state boarding school) provided the courses match up.
One of the newer IB schools, keen to build up their IB numbers, might also consider it, but it would mean contacting each one.
