You can request for rectification of your initial IPv6 allocation/assignment if it no longer satisfies your needs, the same address block will be “upgraded” to the new required prefix size.
However, if the adjacent prefixes are already being used by other organizations or if making the allocation would not leave sufficient space for subsequent allocations, then you can choose to receive a new block.
Take note that you can request the rectification of the IPv6 PA space only once.