Yes. Often, reactions to a proposal surface good improvements that can be used to modify the proposal instantly. The only caveat is that groups are encouraged not to spend too much time perfecting the proposal (since that’s most likely not time-efficient), and that tweaking the proposal too much can water it down or make it incoherent — often, groups start rushing at the end and might add amendments without thinking them through in the context of the whole proposal. Therefore be extra cautious when amending the proposal “on the fly”. Also, be sure that every circle member is aware of the modifications before consenting.
For more information, check out our handbook Many Voices One Song sections 3.3 and 3.5.3