Birth plans are not legally binding. They simply state your preferences for labour and childbirth. You can update and change your Birth Plan before you go into labour. Similarly, nothing in your Birth Plan precludes you from changing your mind during labour (eg you may have initially planned on having a birth without pain relief, but may later choose to have an epidural).
While your Birth Plan isn’t binding, the wishes set out in it should be respected and taken into consideration during labour and childbirth, especially if you previously discussed them with your doctor or midwife.
Even if your Birth Plan sets out wishes that are not what your doctor or midwife recommends, they should respect it and try to meet your wishes (as far as possible). Further, your Birth Plan may be used as evidence in court outlining your wishes and the informed decisions you took when it was made.