It is possible with the kalmar union script.
That requires a spare faction, which you should add on this way in descr_strat
Code:
faction poland (You can used any faction here, but I use poland), balanced smith
ai_label default
dead_until_resurrected
denari 20
denari_kings_purse 100
The denari and kings_purse don't matter, only the part in red. Also you should add the faction in the nonplayable section at the top of descr_strat.
Then you can use a simple script to change the banner:
Code:
monitor_event FactionTurnStart FactionType portugal
and I_TurnNumber = (whenever you want)
set_faction_banner
faction portugal
banner poland
end_set_faction_banner
historic_event portugal_banner factions { portugal, }
terminate_monitor
end_monitor
Then you also need a bit of text in historic_events
Code:
{portugal_banner_BODY}Whatever text you want
{portugal_banner_TITLE}Whatever title you want
And then you should of course set the new faction symbol for the spare faction
and you should use kingdoms, otherwise it is not possible
Good luck!