Hey I use this so happy to give my input.
Background: user selects drop down option at registration. I condition off this. e.g. 'we are': A, B, C.
Auto action
Type: usergroup
Access: everybody
User: Automatic
Trigger: onaftersaveuserregistration...could use alternatives..
Condition: [cb_weare] equal to A
ACTION
1. Mode remove. Remove registered.
2. Mode add. Add usergroup x.
Took me a while to relaise you could use mode remove. That's it. Works for me anyway.
Edit: reread, maybe your looking for something different, more specifici.e. you moved and want to move again based off usergroup value. Useful anyway maybe.
Joomla 3.6.2
CB 2.0.15 Latest Nightly
GJ 3.0.1 nightly
Php version 5.5.35
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Activity nightly
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CB Gallery 2.0
.....all nightly!
Uddeim 3.9