The municipal status of Ko Samui seems to become a quite odd story. In March the provincial governor tried to block the upgrade, but just few days after that was reported it was decided by the Ministry of Interior to upgrade it to city status, effective April 19. However I haven't found any other reports on this thereafter. Samui Express changed their website in April and May making some editions never appear online, so I thought I just missed the important article. Now, to my surprise, on the Thai Wikipedia the article on the municipality was just recently moved to เทศบาลเมืองเกาะสมุย - Thesaban Mueang (town) instead of Thesaban Nakhon (city), one status lower than originally decided. So I asked the one who moved the article - a wikipedian who works a lot on the districts and other administrative entities in the Thai Wikipedia - and he pointed me to a news article of the regional office 5 of the Public Relations Department "ยกฐานะเทศบาลตำบลเกาะสมุยเป็นเทศบาลเมือง", which states that the upgrade was effective June 9. Also the website of the municipality now calls itself Thesaban Mueang.
Neither that user nor me yet know when and why the original decision was revoked and changed; the website with the meeting transcripts right now ends with the meeting on May 8, so maybe after the next update of that site we can see in more detail what happened.