Combining fields from different channels- can I do it and not lose data?

I have two channels with hundreds of entries.

Something like: Channel-1 (1000 Entries) Channel-2 (1000 Entries)

… both channels have each a separate field-group associated with X different fields in it.

Something like: Field-Group-of-Channel-1 (with fields no. 01 to 50) Field-Group-of-Channel-2 (with fields no. 51 to 100)

I now create a New-Field-Group-for-Both-Channels containing ALL the fields from both groups above in this one new field-group (= fields 01 to 100).

Is it possible to now simply go to the settings of Channel-1 and * unselect :checkbox-empty: Field-Group-of-Channel-1 and * select :checkbox-checked: New-Field-Group-for-Both-Channels, *save

and do the same in the settings of Channel-2 (→ unselect :checkbox-empty: Field-Group-of-Channel-2, select :checkbox-checked: New-Field-Group…) … all without loosing any data in the channels/entries? (since the populated fields themselfs are still the same)

And, actually, the same question → but if you select fields individually instead of with a field-group via the Field-Tab in the Channel-Settings?