I’d say Mode 3/Situationally Permissive.
There should be no mental block against sex reassignment in Clan society, for a couple reasons.
One, trueborn phenotypes have displayed characteristics that cross gender norms for centuries: elemental women that have the large muscle-bound physiques of typically male linebackers or body builders and male pilots that have the slight gracile physiques of typically female long-distance runners or gymnasts. These gender-crossing physiques are also widely distributed in the freeborn population as trueborns test out of the warrior caste, enter other castes, and have children naturally.
Two, sex identity has been completely divorced from reproduction among trueborns. Biological males are routinely gene-mothers and biological females are routinely gene-fathers.
With that societal background, sex reassignment would seem pretty pedestrian to the Clan mind. The question is whether the stratified Clan medical system, where access to more and better types of care is determined by your value to society and productivity, would restrict access to sex reassignment treatment based on social status. A highly valued warrior or highly productive scientist or merchant probably has access to treatments, maybe including sex reassignment surgery and hormones, that a slacker or physically aged technician or laborer does not have access to. The canon doesn’t tell us exactly where these lines are drawn, so we’d have to make it up.
It’s also possible that these lines are drawn differently in different Clans depending on how they treat their lower castes. Wolves might make more treatments available to a wider range of castes than, say, the Falcons and Jags.
The same should be true of other gender-crossing expressions. If someone wants to cross-dress, for example, the primary obstacle is probably the lack of resources allocated to personal expression at all in Clan society, rather than any mental block against such behavior in a society of female elementals and male gene-mothers. Outside of warrior caste ceremonies, everyone seems to wear the same unisex fatigues, overalls, work suits, etc. in Clan society (and in BT in general outside the nobility). It’s unclear whether fashion, makeup, etc. are even things in Clan society, but to the extent they are, the more valued or productive members of higher castes will probably have more access to them than the less valued or less productive members of lower castes.
Hope this helps.