If you don't want to tear bridge apart, then you can change properties of existing item using
set command.
I.e. if you have
/interface/bridge
add name=bridge
/interface/bridge/port
add bridge=bridge interface=ether1
and you want to set pvid for port ether1, you can do it like this:
/interface/bridge/port
set [find interface=ether1] pvid=666
And you do it for the rest of bridge ports. After you're done, enable
vlan-filtering=yes on bridge ... note that without appropriate preparation you are likely to lock yourself out of device by doing it. Without seeing full config (prior to enabling vlan-filtering but after setting pvid values) it's hard to tell what you need to add/change not to lick yourself out.
Or are you asking something else?