Rapid changes in all areas of the economy and an accelerated development of new sectors, especially those related to information and communication technology, have created the need to revise Poland’s classification of economic activities. The new revised classification, known as NACE rev. 2, reflects the growing importance of modern technologies and the role of a knowledge-based economy. At the same time, the new system is incomparable with the previous classification, called NACE rev. 1.1. This article aims to develop a method for converting data on employment from the NACE rev. 1.1 to the NACE rev. 2 systems. The study uses LFS data for the 1995–2007 period (NACE rev. 1.1) and the 2008–2019 period (NACE rev. 2). A relationship (transition) scheme was constructed between NACE rev. 1.1 and NACE rev. 2 sections based on data published by Poland’s Central Statistical Office (GUS) as well as a classification of the labour force by four-digit NACE codes in both systems and expert knowledge. Finally, we obtained time series data on the number of employees for the last quarter century (the 1995–2019 period). The series is homogeneous and consistent with data published by the Central Statistical Office on the number of employees in three sectors of the Polish economy in all studied years.