When these areas ‘converted’ to Theravada (which mainly occurred around the 11th-12th Centuries), it is impossible that all the monks took new ordinations. Of course, the official histories will assert that when the religion was reformed that all the monks conformed to the new system. But the practicalities of this are absurd: sending city administration monks wandering through 1000s of miles of tiger-stalked, bandit-infested, ghost-haunted jungle tracks seeking out countless little villages, trying to persuade senior monks that their ordination is invalid or improper and must be done again, all on the basis of some political compromise in a far-distant capital, in a region of ever-shifting borders and allegiances. As history this is sheer fantasy, and the reality must have been that the reforms would directly affect only certain central monasteries.
This book looks like it has a lot to say about tradition, especially in the context of the Theravada bhikkhuni lineage. Definitely a book I’ll be checking out soon: Sects and Sectarianism: The Origins of Buddhist Schools.