Well, I've never heard of Dirk Gently...I know the man for THHGTTG and his aforementioned Doctor Who work, as well as his assorted quips here and there on Atheism, the Universe, and Everything.
But I actually like the fact that THHGTTG has similar-yet-distinct radio, book and TV iterations (the less said about that movie the better, though Alan Rickman was a good casting choice for Marvin and...well, between Arthur Dent, John Watson and Biblo Baggins, Martin Freeman seems to have found his niche in acting every Slightly-Above-Average British Man Flustered and Over His Head part out there.)
There are different versions of Shakespeare plays, of course, some only mildly different, others wildly so (Hamlet's First Quarto's rightfully called "The Bad Quarto," for featuring "To be, or not to be, aye there's the point, To Die, to sleep, is that all?" ...Not quite the refined words you're used to, lol...and then King Lear ends with either Edmund or Albany on the throne, depending on which version you pick, and that's a major choice) and Dickens wrote an alternate end for Great Expectations when his original one was deemed a bit too downbeat.
As long as the essence is intact--ie, the work doesn't become Lucas-ized, with Jar Jar Binks replacing Falstaff or Estella and Pip having a big wedding and living happily ever after--I don't have problem with tweaks after the fact.