Henderson wore No 30 last season following his £15million move from Nottingham Forest but ended the campaign as the first-choice stopper at Selhurst Park.
After being sidelined with a hamstring injury, Henderson started 18 Premier League matches between December and May, while Johnstone featured in just four top-flight games following the turn of the year.
Henderson's promotion to No 1 has seen Johnstone downgraded on Palace's list of squad numbers to No 32, which had been vacant since Levi Lumeka left the club in 2019.
Johnstone took to Instagram on Wednesday to share his new squad number with his 348,000 followers.
Johnstone posted a selfie of himself wearing his new number on a training shirt and wrote: 'New day new number', followed by four laughing emojis.
His post also included his new number surrounded by a circle that flashed in bright colours as he sarcastically suggested that he was excited by the change.
Johnstone has been at Palace since joining from West Brom in 2022.
Both stoppers have also represented England at senior international level. Johnstone has four caps to his name - three in 2021 and one last year, when he kept a clean sheet in a 1-0 win over Australia.
Henderson went to Euro 2024 with England but did not feature in any games. His one and only cap to date came back in 2020 when he played 45 minutes in a 3-0 victory over Ireland.
Reacting to his downgrade from No 1 to No 32, Johnstone posted: 'New day new number'
Palace finished 10th in the Premier League last season after winning six of their last seven fixtures.
The Eagles will begin the 2024-25 campaign on Sunday with a trip to Brentford.