Some interesting facts about the re-conversion of those Hindus who were converted by force by the invading mlecchas (eaters of cow-flesh, those having degraded speech ie anārya- bhāṣā) with the prāyaścittatattva citing the Harivaṃśa on process applied for Yavanas, Śakas, ...

Kambojas and other mlecchas for their re-conversion as well ass detailing how to deal with those who were forcibly converted and made to commit sins (from Śastric PoV) w/ Viṣṇudharmottarapurāṇa taking into account the previous known Varna of those forcibly captured with...

the Brāhmāṇas expected to undertake a half krccha penance in addition to reperforming an ūpanayan rite. In accordance to Varna, a gradual decline in degree of 'expiation' rites were expected. Parāvartana model was followed for bringing back those who involuntarily or voluntarily

left the Hindu fold during a 'Reawakening' movement oft-known in the 19th and 20th centuries as Śuddhi. Interestingly, the author, that is P.V. Kane encourages this process for the 'stability of Hindu society'. He adds that in yore, such people were taken into the fold by...

the performance of a Vrātyastoma rite. Apparently this was the mechanics of spreading Hinduism in the East Indies and Indochina. (I will update this thread at a later date). @sukarma000 this will be of particular interest to you and I will post the theoretical mechanics here: