trigger NewOwner on Opportunity (before update, after update) 
{
//string profileid =UserInfo.getProfileId();
ID oppsid = Trigger.old[0].Id;
User[] PreviousOwner = [select id, profileid from User where id=:Trigger.old[0].OwnerId Limit 1];
//String OwnerId = user.Id[0];
if(PreviousOwner[0].ProfileId =='00eT0000000dnyE')
{
// Territory of the Owner
UserTerritory[] OwnerTerritory = [select Id, TerritoryId, IsActive from UserTerritory where UserId=:Trigger.old[0].OwnerId and IsActive = TRUE];
for(Integer i=0; i<OwnerTerritory.size(); i++)
{
// Parent Territory of the Owner
Territory[] OwnerParentTerritory = [select Id, Name, ParentTerritoryID from Territory where Id=:OwnerTerritory[i].TerritoryId];
for (Integer j=0; j<OwnerParentTerritory.size(); j++)
{
// Get the New User's Id and Title.
User[] NewOwner = [select Id, Title from User where id=:Trigger.new[0].OwnerId Limit 1];
// To get the New User's Territory
UserTerritory[] NewOwnerTerritory = [select Id, TerritoryId, IsActive from UserTerritory where UserId =:Trigger.new[0].OwnerId and IsActive = TRUE];
Territory[] NewOwnerParentTerritory = [select Id, Name, ParentTerritoryID from Territory where Id=:NewOwnerTerritory[0].TerritoryId]; 
for(Integer k=0; k<NewOwnerParentTerritory.size(); k++)
if(NewOwner[0].Title == 'Special' && NewOwnerParentTerritory[k].ParentTerritoryID == OwnerParentTerritory[j].ParentTerritoryID)
{
UserTerritory[] OldOwner = [select TerritoryId, UserId from UserTerritory where UserId=:Trigger.old[0].OwnerId and IsActive = TRUE];
for(Integer l=0; l<OldOwner.size(); l++) 
{ 
for(UserTerritory OldOwnerTerritoryUsers:[select UserId from UserTerritory where TerritoryId=:OldOwner[l].TerritoryId and IsActive = TRUE])
{
OpportunityShare oppsshare = new OpportunityShare();
oppsshare.OpportunityId = oppsid ;
oppsshare.UserOrGroupId = OldOwnerTerritoryUsers.UserId;
oppsshare.OpportunityAccessLevel = 'Read'; 
try
{
insert oppsshare ;
} 
catch (Exception e)
{
System.debug(e.getMessage());
}
}
}
}
/*else
{
Trigger.new[0].OwnerId.addError('Deal can only be assigned to User in your parent territory and with title as Special');
}
*/
}
}
}
}
}