In 1969, PKP started testing several changes to enable EU05 locomotives to achieve speeds up to . First trials were made with the EU05-29 locomotive, and soon it became clear that it was possible to safely achieve speeds up to after changing the transmission rate. The decision was made to rebuild all locomotives of EU05 class during first servicing. As a result, by 1977 almost all locomotives had been rebuilt to EP05 class, except for locomotives 09, 12 and 19, which had been scrapped earlier. To distinguish the new series from its predecessor, the color scheme was changed from two-tone green to orange. All modifications were conducted in "Zakłady Naprawcze Taboru Kolejowego" in Gdańsk.