I assume the question is rhetorical. But as there are no provisions that outline the process within the EASA regs, it is not allowed. Therefore such a procedure would necessitate an addition/amendment/supplement to EASA FCL which regulate such a conversion or license exchange based on a previous license.
if enough license holders petition the UK government, as previously suggested, then perhaps the UK can start to negotiate such an amendment or dispensation with the EU (EASA)?