The two events from PON that are relevant here are this one:
SelectFaction = CMN
SelectRegion = $Brandenburg
SelectFaction = CMN
SelectRegion = $Brandenburg
StartEvent = evt_nam_PRU_NorthGermanConfederation1867|1|2|evt_txt_PRU_NorthGermanConfederation1867|Event-img_GER_NorthGermanConfederation1867|$Brandenburg|NULL
Conditions
SelectFaction = GER
EvalIsAtPeaceWith = AUS
MinDate = 1860/01/01
MaxDate = 1875/03/10
Probability = 25
EvalEvent = evt_nam_GER_Hannover_UnificationOK;=;1
EvalEvent = evt_nam_GER_Sachsen_UnificationOK;=;1
EvalEvent = evt_nam_GER_Hessen-Kassel_UnificationOK;=;1
EvalEvent = evt_nam_GER_Hessen-Damrstadt_UnificationOK;=;1
Actions
DescEvent = evt_desc_PRU_NorthGermanConfederation1867
AbsorbFaction = HAN;100;100;100;100;100;100;100
AbsorbFaction = SAX;100;100;100;100;100;100;100
AbsorbFaction = HEK;100;100;100;100;100;100;100
AbsorbFaction = HED;100;100;100;100;100;100;100
AbsorbFaction = HLS;100;100;100;100;100;100;100
ChgFacDisplayTag = NGF
ChgVPCount = 100
ChgRgnDecisionFP = $rgdProposeUnificationGER;3
EndEvent
and this one:
SelectFaction = GER
SelectRegion = $Brandenburg
StartEvent = evt_nam_PRU_Germany1871|1|2|evt_txt_PRU_Germany1871|Event-img_GER_ImperialGermany1871|$Brandenburg|NULL
Conditions
EvalEvent = evt_nam_PRU_NorthGermanConfederation1867;=;1
MinDate = 1870/07/01
MaxDate = 1888/03/10
EvalRgnOwned = $Alsace
EvalRgnOwned = $Lorraine
Probability = 85
Actions
DescEvent = evt_desc_PRU_Germany1871
AbsorbFaction = BAD;100;100;100;100;100;100;100
AbsorbFaction = WUR;100;100;100;100;100;100;100
AbsorbFaction = BAY;100;100;100;100;100;100;100
ChgFacDisplayTag = GER
ChgVPCount = 1000
ChgRgnDecisionFP = $rgdExploreParty;2
ChgRgnDecisionFP = $rgdNatExpedition;1
ChgRgnDecisionFP = $rgdGeoExpedition;1
ChgRgnDecisionFP = $rgdAnthroExpedition;1
ChgRgnDecisionFP = $rgdProspection;5
ChgRgnDecisionFP = $rgdProsecute;3
ChgRgnDecisionFP = $rgdPacify;5
ChgRgnDecisionFP = $rgdRetaliate;5
ChgRgnDecisionFP = $rgdOutpost;10
ChgRgnDecisionFP = $rgdGunboatDiplomacy;1
ChgRgnDecisionFP = $rgdNavalDemo;1
ChgRgnDecisionFP = $rgdChiefBribing;10
ChgRgnDecisionFP = $rgdChiefTreaty;3
ChgRgnDecisionFP = $rgdChiefTerritory;3
ChgRgnDecisionFP = $rgdMissionary;5
ChgRgnDecisionFP = $rgdMerchants;5
ChgRgnDecisionFP = $rgdTradepost;15
ChgRgnDecisionFP = $rgdRoad;10
ChgRgnDecisionFP = $rgdTelegraph;1
ChgRgnDecisionFP = $rgdSchool;5
ChgRgnDecisionFP = $rgdVaccination;0
ChgRgnDecisionFP = $rgdSanitation;0
ChgRgnDecisionFP = $rgdImmigrants;10
ChgRgnDecisionFP = $rgdSociety;1
ChgRgnDecisionFP = $rgdProtectorate;2
ChgRgnDecisionFP = $rgdColony;2
ChgRgnDecisionFP = $rgdStake;10
ChgRgnDecisionFP = $rgdCustoms;1
ChgRgnDecisionFP = $rgdRail;1
SetEvtOccurs = evt_nam_PRU_SouthGermanConfederation1872;MaxOccurs;0
EndEvent
The nation (GER) switches from its old flag to a new one called NGF
ex:
GER NULL ChgFacDisplayTag NGF to display the NGF flag (instead of initial one)
See also the wiki here
http://www.ageod.net/agewiki/ChgFacDisplayTag