Is it permissible to alter a vow related to masturbation by increasing the number of rak'ahs and parts of the Qur'an vowed to be performed, and what is the correct formula for doing so?

Enjoying one's wife in a way that does not involve the forbidden act of masturbation is permissible. The emission of semen as a result is not problematic and does not fall under the vow. As for changing the vow, you are only bound by what you vowed. This vow of yours is a vow of obstinacy (nadhir al-lajaj), and you must either fulfill it or offer an expiation for an oath. You must abandon masturbation in obedience to Allah. We do not advise you to make vows to refrain from disobedience; rather, frequently reflect on Allah's names and attributes, on death and its horrors, and remember His constant awareness of you.