In "olden" days, .357 revolvers often, maybe mostly, had cylinders slightly longer than .38 cylinders, so the jump to the forcing cone was greater for a .38 fired in a .357 than a .38 fired in a .38. For a number of years that has not been the case with most, if not all, revolvers. Smith and Wesson went to the same length cylinder for .38s and .357 a number of years ago and Rugers are the same length also (though they offer few dedicated .38s). Perhaps there is some velocity/accuracy loss due to the little bit of "freebore" when a .38 is fired in a cylinder cut for .357 (I don't know) but accuracy loss due to the jump from cylinder to forcing cone is not an issue on recent revolvers. At least, I don't think so.
Use it up. Wear it out. Make it do, or do without.