After months of waiting and following up with Gumi support, I can confirm that it is possible to swap your linked FB account now. Unfortunately, I can't say for certain what exactly caused them to change their mind. However, they did confirm that I could share my experience with Reddit, so here is a run-down of what I did. I don't know if this will work for everyone and it may be a case by case basis. I doubt they will allow this for account rerolling.

Warning: A text wall approacheth...
Around mid-July, I contacted support asking about switching my account that I had previously linked to FB. Basically, started playing. Linked to FB Didn't like it (PAD life at the time). Deleted the game. Couple days later, IRL friend encouraged me to start again and offered to carry me. Downloaded the game again and started over. Got to Rank 20 before I realized the game wouldn't let me link my current game to FB and wanted to restore my Rank 5 one.
Support gave me the canned 'no-can-do' answer. After looking through the Facebook Developer agreement and their privacy policy, I rebuttaled with some quotations from those agreements, but still CS said it couldn't be done.
Then I made a Personal Information Request by terms of their privacy policy to their legal department in Shinjuku by snail mail. Also looked up a email address contact through the BBB.org and wrote an email expressing my concern.
About a week or two after that, I received a new response in my ticket from the support team that they needed more information. Standard lost account info - ID, Support Code, In-game Friend names, Summoned characters, etc. They confirmed the info and said it was being passed up to the Dev team. Two weeks later, no response, asked for a follow-up from the Dev Team. CS said it was still being worked on. A month after that, I asked again for a follow-up and cited that I was a bit nervous to spend money in the game without having a solid method of backup (although I had been using TiB jic). At this point I was about Rank 70 in the game.
Got another reply asking to confirm the player IDs that I wanted to switch and warned me that the other account wouldn't be playable anymore. Said they completed the switch... Checked with the FB login; still wanted to sync my Rank 5 account. (Are you trolling me Gumi?) Contacted CS again and about a week later, they apologized and asked me for screenshots of the Player IDs. Then they asked me one more time, are you sure about this? Confirmed and within an hour, I was switched! Logged into Facebook in game on my phone and it asked me to sync my Rank 74 account! Success never felt so sweet!

TL;DR - Be persistent, be polite and try every angle. FB linked account switching is possible!