Tuning down can help, if an old guitar is about to fly apart, or if the neck is too soft... It is easier for your fingers, and it can get deliciously basie! If you want to go even further down, you just need to string up a heavier gauge of string... but probably best not to do this if there is a chance someone will tune it back up to standard tho...
DADGAD is fairly easy to learn, and since I went there, I have a really hard time going back to standard! Just start by playing songs you already know, and the new fingerings will begin to make sense. Though Db is a rediculously hard key to play in, Em, F, F#m, G, Gm, Am, A, A# Bm, Cm, C, D,and especially Dm, are relatively straight forward! Just tune it there, and give it a full day or two. You might not go back! I didn't! I personally think that it is a good idea to try Keys other than "D" in DADGAD... cause it is SOOOOO easy to play in "D" once you've got it a bit!ttr