It used to be in EE2 you could move an entry to another channel that shared the same fieldset. I can't seem to do that anymore in EE5 (Options tab > Channel). Does anyone know how I can change an entry's channel definition in EE5? I can go into exp_channel_titles I suppose if I need to, but maybe there's a more intuitive way?
In EE5 you are able to assign other defined fields in addition to a field group. In EE2 you can only assign a field group to a channel. I think this could be the reason you can't reassign an entry to another channel.
If your channels have the same fields, then you can try doing the change in the Channel Titles table.
Hope this helps.