If you had attained anariya jhana BEFORE becoming a Sotapanna, then it would be EASIER to convert that to an Ariya jhana.
But I don’t think it will AUTOMATICALLY turn into an Ariya jhana due to the following reasons.
1. Kama raga is only temporarily SUPPRESSED in the first anariya jhana. Thus, even though one with that WILL BE born in a lower Brahma realm upon death, that Brahma WILL come back to the human realm and could be born in an apaya in future lives. That is because kama raga anusaya remains intact in an anariya jhana.
2. One gets to the FIRST Ariya jhana by REMOVING kama raga, i.e.,kama raga anusaya is not there even in the FIRST Ariya jhana.
– Thus, if one gets to the First Ariya jhana one is essentially an Anagami.