The .30-30, .38-55 and the .375 Winchester share the basic case diameter and rim dimensions. The .30-30 was derived from the .38-55, and the .375 Winchester is a modernized, improved version of the .38-55 with more consistent barrel bore dimensions.
If your conversion is a lever action .30-30 to .38-55, it can be easily done by re-barreling and at the most, changing out the left and right cartridge guides on an M1894 Winchester. It is even easier on a Marlin rifle.
As for converting those same lever actions to .375 Winchester, it is not advised, because the larger cartidge has considerably more breech thrust than the original .30-30. A responsible gunsmith would not do it.
A single shot .30-30 is likely a good candidate for conversion to either of the larger calibers, and is easily done by re-barrelling. The .38-55 is particularly favored by the BPCR shooters for mid-range competition shooting.
