No change in doctrine? Read my tag line. Please tell me what pre-Vatican II teaching ever even suggested that non-Catholic churches were “means of salvation”.
And that’s just one example.
First, that pronouncement would need to have been ex cathedra and VATII contained no ex cathedra statements.
Second, the old Baltimore Catechism described three types of Baptism, only one of which was by water (the most easily recognized “outward sign” and practiced by nearly all Christian, nonCatholic sects). The other two were listed as “by fire or blood” and “by desire.” Fire or blood covers individuals who, never having been baptized by water, gave their lives for their (Christian) faith (think martyrs who haven’t finished instruction); and desire covers individuals who, through no fault of their own and while striving for goodness/holiness in their lives, were never exposed to Catholic or even Christian teaching (think remote locations, lack of education, etc.). Thus, it would follow that any entity, “church” or otherwise, that promoted adherence to natural and/or God’s law in the absence of access to knowledge of Christ and His Church, COULD be a “means of salvation.”