What part is complicated when transitioning such a system? Legacy data which must be transformed? Old business logic which must be maintained? I do not know i

What part is complicated when transitioning such a system? Legacy data which must be transformed? Old business logic which must be maintained? I do not know if there is a need to replay old rules vs. just kerping a snapshot Continuity? Maybe there are so many transactions happening, linked to existing data that an interruption is complicated? Risk of accessing old data which is better left undisturbed?