In music generally : songs still sound good if you assign parts to Different Instruments. Especially if you personally love those Different Instruments.
That's because the music theory, the 'rules and conventions' under which almost all western music is written, is the thing which makes a composition "work" to our ears.
The specific /instruments/ that are assigned to the composition absolutely can be changed, and it will still be the same coherent piece of music which adheres to all the same rules in the same ways.
To put yourself in the composer's shoes and say, "well I believe *I* know better than the original composer, and believe if I changed this instrument or that, and took that one out entirely, and so on and so forth.... then I end up with something even better-sounding", then that is perfectly valid, for you.
It's the same as somebody obsessed by Hammond Organs might say "well, I far prefer Black Sabbath's music when all of the guitars are instead changed into Hammond Organs", and yes, for a person who loves Hammond Organs to Excess, then that undoubtedly would be true, for them.
But I believe a certain respect is also necessary, recognising that the decisions made by the original composer are part of what defines that composer's work, and once you start mucking around with that, then you are not "making that person's work better" in any objective sense.
Rather, you have forked (in programming terms) off into your own microcosm.