I have been giving 10% on my gross pay for some time. I read in one of your answers that 10% is usually given on net earnings and not on gross. Is it permissble for me to change to net even though I have been giving on gross?

Even though Maaser Ksafim (10%) is of rabbinic source this can be a Torah level (Deoraitah) issue of Nedarim (vows) If you therefore want to change your amount you give for Tzedakah – you should have the Hatarat Nedarim made first by a Rabbi or a Torah scholar