Per Orach Chaim 693:2 if one forgot Al Hanisim by Davening there is no need to repray. What if I said Al Hanisim, however I said the Chanuka version on Purim - do I have to Daven again or not? ...
In OC Siman 695 Sif 3 that if one starts their Purim seudah by day and finishes by night they should still say "Al Hanisim" in Bircas HaMazon. The Machaber brings an opinion that says to not say it; ...