Q: To what extent is sensitivity affected if I switch to methanol? Do I have to remake the calibration curve?

A:

The sensitivity does not deteriorate greatly if you replace the acetonitrile in the mobile phase with methanol.
Note, however, that with methanol, absorbance at the short wavelength end of the UV region is large in comparison with acetonitrile, and depending on the detection wavelength, the baseline may be difficult to stabilize or there may be a large amount of noise, and this may adversely affect high-sensitivity analysis. In gradient analysis, ghost peaks are larger with methanol than they are with acetonitrile.
In LCMS analysis, the ionization efficiency, and consequently the sensitivity, may be affected. (Depending on the compound, the ionization efficiency may be higher with methanol than it is with acetonitrile.) The analytical conditions change greatly and so remake the calibration curve.