I have channels sharing the same field group and am thinking about re-assigning them to different groups because they will develop in different directions (= different fields needed). Is this possible and what would be the best approach? Thanks a lot for suggestions!
If you've already got a lot of entries published in any of these channels I'd recommend not changing the field group for those channels. The reason being as soon as you go to edit one of your existing entries you will not see the custom fields where you data was stored and as soon as you resave the entry you will lose the data that was stored in the custom fields in the old field group.
So instead just add any new fields to the existing field group to accommodate the different directions your channels are headed in. Then use Publish Layouts on a per-channel basis to hide any irrelevant fields.
If you really need to change the field groups and you have a lot of entries you can migrate the existing field data to new the new fields. You'd need to create analogous custom fields in your new field group and then use SQL to copy the data over to the new fields in
Here is some SQL to use as a starting point:
UPDATE exp_channel_data SET field_id_X = field_id_Y WHERE channel_id = Z
X = the new field ID number
Y = the old field ID number
Z = the channel ID for the channel at hand
I recommend using a tool like Sequel Pro to help you inspect the data and run the queries. Make sure you backup your database first!