Having it once won't hurt your gains and pumps some more cals into daily intake which, sounds like you need. Don't think about it, just eat and move on, key is not to repeat it.
As far as your changing routines this is MO.... Your coming into summer I always like to cut in summer, lose some pounds aim for the beach bod so to speak.
So cutting for the next few weeks (6-8) should get you down to a nice base to start 5/3/1 for the winter cos as you say it takes time.