In 2019 the Finnish strong eID services provided by banks moved from a legacy protocol (from the early 2000's) to OpenID Connect. Other strong eID providers switched to OpenID Connect as well, and eID brokers started to gain traction.
The Finnish Trust Network consists of 13 commericla IdPs and 1 govt IdP, providing strong eID services to both public and private service providers. Before the adoption of the trust network, each service provider had to establish a commercial agreement and a technical integration with each bank separetely (10 banks) and one mobile network operator (3 MNOs providing eID services) for the SIM based mobile ID services. The brokers simplified the process by allowing service providers to acquire all identities issued by the IdPs through a single commercial contract and technical integration. Some provieders function as issuers and brokers, some are solely issuers and some only brokers.
How did we manage to switch the whole country, over dozen IdPs, hundreds of service providers from a tried and tested and simple old protocol to Open ID Connect? Why did we do that? Was it smooth? What are the lessons learned and what is needed from OAuth/Open ID Connect now?