It all depends on what the current string gauges are, and the VSL -- the Vibrating String Length, or distance between the nut and bridge. Unfortunately "extremely taut" is sort of subjective. What's extremely taut to you might not be to someone else.
Tell us the VSL, and ask your f-i-l what gauges of string he had on the dulcimer when he gifted it to you.
Once you know the VSL you can go to the Strothers String Gauge Calculator, enter the VSL and choose "d" not "D" for the note you wnt to tune to. The calculator will tell you what gauge the string should be for that note and that VSL. Be sure to select "d" not "D". Most people today write DAd rather than DAD, because that tells the reader that the bass string D and the Melody string d are an octave apart -- the melody string being higher in pitch.
Usually a melody string that can tuned to A can also be tuned to d, although it certainly will be more taut at d than at A.