เทศบาลตำบลป่าสัก) was announced in the Royal Gazette. The change became official on November 6, exactly 4 years after the last local election - even though all local elections are still halted and mayor can continue to stay even after their normal term in office ends, in this case the Ministry of Interior timed the update with the end of the electoral term, same as they did in past when it still made sense to avoid too frequent local elections.
The upgrade was discussed in the Board to consider draft laws in their meeting on September 7, thus it was rather quick to be announced in the Gazette given that there are still hundreds of upgrades still not yet published there, waiting for more than eight years already. Maybe this time it was so fast because it was the first and so far only one planned for the fiscal year 2016, and even just the third for the common year 2015.
Though there is no Wikipedia article yet about this local government unit, I have created and filled an item in Wikidata to record the information - and since it is a reference for some of the statements in that item, even this Royal Gazette announcement now has its own Wikidata item.