I sent my 3rd Generation SAA (45LC) to Colt and had an ACP cylinder fitted to it. That was back in 2000. Had the gun re-blued while they had it. As best I can remember, the whole package was around $250. I'm sure prices have gone up on various option ~ but you could take off the re-blue unless you needed it.
Just an option if you can find the right gun at the right price.
Best money I ever spent. The 45 ACP has saved me enough to justify the cost and personally, I think the 45 ACP is more accurate out of my gun.