Controlled vocabularies and IT systems enabling their use have been in the forefront of library work for decades. In the National Library of Finland the national bibliography has been indexed using the YSA general thesaurus since the 1980s. A dedicated browser called VESA was developed in 1999 in order to eliminate the need to publish YSA as a printed document. In user surveys, VESA continually ranked as our most loved service. However, as years went on it became more difficult to integrate VESA’s old code to new environments. When the time came to renew VESA, library world was already buzzing with open linked data, semantic web etc. So it was decided that the new system should provide YSA and other vocabularies as open linked data with the ability to integrate the vocabularies to other systems using modern APIs. In 2013 work begun on the national ontology and thesaurus service Finto slated to replace VESA. Due to VESA being so well-liked, Finto was developed in deep collaboration with the users. Regular usability tests were conducted during the development and in all aspects and features care was taken in order to not put any extra burden on the daily tasks of the annotators. Finto provides the functionalities that VESA did, but also offers various new features and possibilities. An example of an auxiliary feature is the new suggestions system streamlining the process of gathering suggestions for new concepts into Finto vocabularies. Furthermore, the modular design of Finto also allowed us to utilize open APIs in other systems to, e.g., provide direct links to content annotated using a given concept in a vocabulary. We present the lessons learned during the development of a replacement for an extremely well-loved core service of a national library. A particular focus will be on the collaboration with the users during the development process and the migration.