Very interesting question Philip.
There is no easy, and for easy read direct contractual, answer to this. There is, for a host of reasons, no mechanism outside acceleration for reducing the time available to carry out the work.
Better than just writing out the cost in the CE equivalent to the bonus would be to make an assumption that the Contractor will finish early as a consequence of the change and therefore lose the effect of the bonus for that period (probably phrased as a reduction in risk) that way the assumption can be revised if necessary. The wording of the assumption, and indeed any deduction would need to be very carefully thought through.
It may also be possible to change the WI to define Completion to hold it back to where it was originally intended to be. That is however very hypothetical and without context may or may not be possible.
Any of these solutions are difficult and challengeable. Much more secure, and certainly the right option, is to have a sensible dialogue around acceleration. Yes the contractor could refuse but it is still the better contractual mechanism, refusal is a commercial issue. The above options are contractually insecure so while they might give the perception of a fair answer you could end up spending a lot on legal costs to find out whether an adjudicator, arbitrator or judge agrees.
Ultimately, if there is no dialogue, the contractor is almost certainly entitled to its bonus and the Employer should take that into account when changing the scope.