The change you're after is a good one to do...same color family, just darker.
If done correctly, and good leather, it should essentially hold the new color practically as long as the original...no reason why not.
If it was a black leather, & you wanted red, I'd caution you about color "lift", where the old color might show in a crease caused later by wear.
Not a likely problem for what you want to do.
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Cost? This would basically depend on the square yards of the leather being dyed, but there is a wide range of charges from different shops.
A truly quality job could easily cost around half of what the piece sold new for...and that's why many people sell what they have, and go get one in the color they like.
But...it CAN be done! Price it out to a couple of shops by taking some pics, measuring it, and taking a cushion (if you can) so they can assess the quality of the leather.
Worth checking out...then you can decide what's best.
HTH
