@joo Really neat idea!
If you look at the plugin now, I think I did this too (was running some final tests). I see you looped through each user, but in big forums, this may take a while.
So, I did a SQL UPDATE:
(I adapted Ask Discourse’s query and modified it)
This way, in the setting tl1_title_on_promotion, you should be able to use something like Junior {group_name}, so then the title would become Junior Developer.
Note that you would also have to tick add_primary_group_title for it to work.