The domain trusts would be unaffected by these upgrades.
--please don't forget to Accept as answer if the reply is helpful--
We are working on Active directory upgrade from 2012 R2 to 2019. There is a trust relationship that exist between the primary domain and other domains (around 15). With some of them the trust is bidirectional and with some inbound/outbound. The concern is when we promote new 2019 domain controllers or demote old 2012 r2 domain controllers will it effect the trust relationship between domains ?