I don't want to sound too cynical here, but what about looping over the text (or array, I suppose the text is stored within the array you insert into the method via the previously mentioned parameter), and replace by index whenever some if condition emits true?
thinks like

arr[0] = 'i';

or

arr[i] = 'u';

and stuff work wonders here .
If thats not what you were asking, i'd recommend to specify your question more clearly, 'cause I seriously don't get what you want right now.
Best Regards.
Hijacker

OK, so maybe I'll explain my idea more clearly.
As you may or may not know, cyryllic alphabet is precursored by something called glagollitic.. This writing method has been used to write old slavonic text, however glagollitic can also be resembled using the today cyryllic alphabet. So I wanted to write the text converter, but... there are about 90 characters, so I really need to write 90 if statements or case blocks?

you can use multiple approaches here. either use an array (as suggested above), or you can use a dictionary to map those characters and do as you like. There are always multiple aproaches to a given problem, they usually only differ in algorithm speed and code elegance.
Best Regards.
Hijacker