Madanm Mandaly, I have run across "fi a" and "fi an". I realize that they are both correct, but are they always interchangeable at random or at will? Then again, which is more preferable? Kreyol definitely has a flavor of its own. Mesi anpil.

I use both.
I don't think it's about preference.

The majority of times people use definite article "an"/"lan" instead of "a"/"la" because that's how they talk in the region they are from.  
We say "Fèmen pòt la" and they say "Fèmen pòt lan" (Close the door).  
We say "Vitès la te ilegal" and they say "Vitès lan te ilegal" (The speed was illegal)
We say "Tibebe a grangou" and they say "Tibebe an grangou" (The baby's hungry)



And, other times a definite article "a" or "la" might be switched to "an" or "lan" depending on nasal sounds preceding or "surrounding" it.

For example:
instead of:
Netwaye kay la ban mwen
I could also say
Netwaye kay lan ban mwen. (influenced by the nasal sounds in ....ban mwen)
even though the rule says that we should say "kay la"

other examples:
You'll say "zanmi an" instead "zanmi a", because of the nasal sound "zan..." in zanmi
You'll say "fanmi an" instead of "fanmi a" because of the nasal sound "fan..." in fanmi.

We say:
jou a or jou an
avangou a or avangou an
bouk la or bouk lan
soukous la or soukous lan
lanmou an because of the nasal sound "lan..." in lanmou

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