Maybe you should ask for refund and book a new ticket if lower booking classes are available (and presumably at a lower fare than what you paid)?

Yes, you do not need to confirm the changes or do the rebooking now.

It has been confirmed in other posts that TK will rebook into lower fare class if available at the time of rebooking and from that it is a fair assumption that TK will also rebook into higher booking class if lower booking classes depleted at the time of invol. rebooking - but I can't recall any recent posts in this forum actually confirming this. It can't be excluded that TK IT is so sophisticated that invol. rebooking will be in the same booking class even if depleted at the time of invol. rebooking, in same way that TK will invol. rebook award tickets into X or I class despite such booking classers not being available.