For the ten months prior to 1.18, THM was arguably the best healer in the game. Their role as a healer was diminished in 1.18, but THM is still a capable healer today. However, in 1.20, it has been revealed that THM will no longer have a native healing spell. Given that THM is also losing Siphon MP (while CNJ receives new MP regeneration abilities), it seems that THM will effectively be transformed into a nuke-only job.

Speaking for myself, I ranked up THM under the old mages-are-good-at-both-roles paradigm, with the intent of using THM as a healer. Now that THM is being transformed into a straight nuking job, I am disappointed to learn my healing class will not be able to heal any more.

I understand that the players voted for drastic changes in the survey. However, if such changes are going to be implemented, is it possible for the players to be able to reassign class levels to match up to the new vision of the game?

This isn't a simple issue of "ARC isn't good any more so I want to be MRD now"; there are always going to be shifts in the pecking order of DDs. This is more like removing ARC's DD abilities and replacing them with BRD songs (which is, notably, not what is planned for ARC/BRD). There's a huge difference between shifts in the effectiveness of a role and removing that role altogether.