Block is used when you don't have a DEX advantage over your opponent. If the PCs have a higher DEX than the NPCs, Dodge is better because they already have one of the main benefits of Block (acting before their opponents).
I seems like magic skill is at best loosely connected to the ability to access magic. Someone could be a fantastic spellcaster, with no RSR and no Magic Skill, while we could have a researcher who understands the theory but lacks any practical ability. With that in mind, perhaps a skill roll to reflect a greater impediment to accessing the underlying magic is not the best simulation.
Not RAW, I believe, but what about imposing a requirement to make a roll for every spell without tying it to a characteristic? Make the base roll 15-. For most heroic games, that is as easy, or easier, to make as any characteristic roll. Now apply the usual cost for penalizing a skill or char roll to reduce this roll by 1.
So if we have our novice wizard who will only make his skill roll half the time, and our master who does not require a roll at all, and they enter a powerful 11- magic nullification field, 11-. each can access the magic if they make the 11- roll, but the novice still has to make his normal RSR roll to sculpt the magic in the manner he desires, while the greater skill of the master means that, if he can access the magic at all, it will do his will.
I'm heavily on the "It depends on the campaign" side of things.
It also depends on the likely duration based on the the reversal condition. Which makes me consider that reversal conditions should probably be figured somehow into the cost of transformation, but that's a different discussion.
If it's going to be reversed by the end of the session, I'd call it cosmetic in any except a very heavy role-playing campaign, and maybe only more then, if the characters have reason to believe it's long-term or permanent.
So, let's assume it's long-term or effectively permanent.
For a game where where non-combat time and role-playing are important, and this is take seriously at all, then this is at minimum a minor, and more realistically a major transformation. IndianaJoe3 mentions social limitations, but the biggest imposed limitation here is going to be gender dysphoria. That could be written up as a psychological limitation, a social limitation, or a combination. It could also be argued as a physical limitation, but that might be a stretch.
And gender dysphoria is very real, and, depending on the person can be crippling. I, luckily, never had a severe dose of it, but I have friends who have. Bought realistically, the lims involved could easily top 20 or thirty points.
And of course, it wouldn't aways be lims added. For a transgender person who hasn't started transitioning yet, it could easily remove a slough of lims. If they have started transitioning, it would depend on what exactly the transform did to them. If it just goes off your genes, swaps some chromosomes, and rebuilds you, I personally would be fine with it. If it's some metaphysical reversal, where I end up physically a trans guy at the equivalent stage of physical transition, I burn down the world to get the reversal or take pretty savage revenge if it's not possible.
I'm pretty confident for most non-transgender people, a permanent change would not be a small thing. It would upend their lives and sense of self. Played realistically the affected person would be more like my second reaction above. They would need to find the reversal. It would consume much of their life until they did. So, in a serious, RP heavy campaign, I really see it as a major transform.
To the recognizability issue, that is variable, but generally the change would probably make pretty big differences. For the closest real-world equivalent of a boy-girl gun, hormone replacement therapy (HRT), results vary from, "yeah, she's looks a little more feminine now" to "There's no way that's the same person." Especially with trans women, because a lot of the changes testosterone makes to the body (height, build, facial features), are at to the bone, and thus not as subject to the kind of changes that HRT can make. An actual boy-girl gun wouldn't be subject to those limitations, so, big, big, changes (on average). Anecdotally, recently while grocery shopping, I ran across some people I gamed with for 8-10 hours every week for years, and they didn't recognize me, even when looking straight at me. And I started HRT really late in life and didn't get the full benefits a younger person would have gotten.