As far as I know Ruger doesn't sell these parts like S&W does.
S&W hasn't sold barrels or cylinders for over 30 years, nearly 40.
I think Ruger sells some parts but I don't think they sell barrels, you'll have to as Ruger.
Best solution, BUY the gun you want, then either keep both, or sell the one you don't want anymore.
Next best, sell the one you've got and buy the one you want, but then you're without a gun until the one you want arrives.
Pretty sure the labor cost (leaving aside the parts cost if you can get a barrel) would be more than the price difference between the short and long barrel model. SO, even if you COULD get the barrel you want, having a smith install it is the least desirable option, economically. And if you need to add in the cost of the new barrel, its way beyond "cheap".
IF Ruger will do it, and you think the cost is fair, have Ruger do it. If not, I think you'd be better off buying another gun with the barrel length you want, rather than trying to get the barrels changed. Its not the 70s or the 50s anymore....damnit...