Officials announced the switch on Thursday. The event is set to take place at O2 Arena in London.
Johnson (14-8) is coming off a second-round knockout over Gleison Tibau at UFC 168 last month. He publicly campaigned for a fight against lightweight contender Khabib Nurmagomedov, but now draws Guillard instead.
A former contestant on "The Ultimate Fighter," Johnson has compiled a 6-4 record in the Octagon. He is currently on a 2-fight win streak.
Guillard (31-12-2) is a former teammate of Johnson's at the Blackzilians camp in Boca Raton, Fla. He left the team prior to a fight against Mac Danzig last July, which he won via knockout.
The bout between Guillard and Pearson was announced in October, immediately after they fought to a no-contest at UFC Fight Night 30. The result came when Guillard landed what was deemed an unintentional knee to a grounded opponent.
Pearson (15-6) revealed it was a knee injury that forced him from the card, via his Twitter account. The 29-year-old went on to say it would require surgery.