Migrate multiple custom fields to just one Fluid Field

Grown with the years, one of my projects consists of several Channels with several Field Groups and dozens of custom fields. Now with ExpressionEngine 4 and the new fluid field we want to consolidate most of the custom fields so we’re not longer depending on so many custom fields … whats the easiest and safest way to migrate from «multiple custom fields to one fluid field»?

For example – right now we have Channel1, Channel2, Channel3 etc. each with field groups with custom fields like channel1-bodytext, channel2-bodytext, channel3-bodytext and of course many, many more.

How can I migrate/export/import those into just one text-field inside one fluid field used by all channels?