Batch / bulk custom field context script

Hello there,
trying to put together the script for cloning custom field context script.
Usecase:
Large customer, 500 projects, about 2000 CF, all with contexts.
Running migration - split of 1 project into 4.
Project using about 80 custom fields, which do have random context (not only to this 1 project).
Need to update those 80 CF - append 4 project context, even the default would work for me.
Listing that tiny window with 5 project and trying to select within 500 projects where some of them are already selected showed as really impossible.
I got this as inspiration SplitCustomFieldContext.groovy | searchcode but generally stuck with getting contexts from cfs.
Did you faced anything similar?
Thanks, Tom