The Roman Catholic Church would remain the firm's principal client until the early 1970s. In 1938, Thomas Warnett Kennedy became a partner with Coia, contributing to designs for St Peter in Chains, Ardrossan, and the Roman Catholic pavilion for the Glasgow Empire Exhibition. The practice also collaborated with Thomas S. Tait on the Exhibition masterplan. Even though the Second World War brought a hiatus in the practice's work, one church that they designed, St Columba's Church in Glasgow, was completed in 1941. The "Dictionary of Scottish Architects" states that Coia was interned as an enemy alien during the war. However, Rodger disputes this, stating that there is no record of the Wolverhampton-born Coia's internment, although several of his Italian-born relatives were taken to the Isle of Man.  Gillespie, Kidd & Coia was revived in 1945, although without Kennedy, who later emigrated to Canada. Coia hired 17-year-old Isi Metzstein as an apprentice and continued to design churches and other works for the Roman Catholic Church. The firm's work of this period is considered by architectural historians to be inferior. Rodger describes Coia's difficulty with seeing projects through, which was countered by his "flair for heading an architectural office". This led to the development of an atelier-style practice, with Coia gradually handing over design control to concentrate on client relations. In 1954 Andy MacMillan, a contemporary of Metzstein at the GSA, joined the firm from the East Kilbride Development Corporation, and despite neither of the young architects being fully qualified, they had assumed creative control by 1956. Coia's last significant architectural work was for St Charles, North Kelvinside in 1959. The first result of the new designers was St Paul's, Glenrothes (1956), which broke with the practice's earlier church designs by embracing the Modernism of Le Corbusier and Frank Lloyd Wright. Their opportunity lay in demographic changes taking place in Scotland at the time. The huge post-war construction project of new towns relocated many people from inner city Glasgow. These changes required new churches for the new town communities, as well as new city churches to service the remaining congregations. Gillespie, Kidd & Coia were one of the few practices involved in the building of the new churches. MacMillan and Metzstein's work in the period 1956–1987 was enabled not only by MacMillan's practical experience at the new town of East Kilbride, but also by the willingness of their principal client, the Roman Catholic Church, to accept radical experimentation in the wake of the Second Vatican Council.