changing your name is incorrect, when you put "Droga" before a name.
WTF?
When you address someone, you say "Droga Estero". When dear Esther is the subject of the sentence, it's "Droga Estera". It's not the matter of whether there is 'droga' before it or not. This is the matter of grammatical case. When you address someone, you use vocative case (Estero, Maćku, Krystyno etc.), and when you talk about something, you use other cases. Vocative case is a bit misleading because adjectives look the same in vocative and nominative case.
If you had 'drogiej', 'drogą', then it couldn't be 'Estero' or 'Estera', but other endings.
It goes like that:
droga Estera (nominative)
drogiej Estery (genitive)
drogiej Esterze (dative)
drogą Esterę (accusative)
drogą Esterą (instrumental)
drogiej Esterze (locative)
droga Estero (vocative)
But "Droga Estero" is absolutely grammatical as long as it's in the right context.
