As a zelda main, I feel the shadow of sheik's greatness, so the benefit of transforming to sheik seems very real. But I don't imagine sheik mains feel the same way. I hope that in the future they design transform to give sheik mains the extra incentive to use transform in battle that they'd need to switch to an objectively worse character, however, there are a few tangible advantages for shiek to transform already.
Boost transform. This is a special recovery technique for sheik. You're already aware that zelda has a longer and floatier recovery, but did you know that with a certain input after hitstun you can do a super air jump during transform? Right when your hitstun ends, press jump and you'll notice a higher than normal air jump. If you quickly press down b (very quickly, you need to claw it) after the jump, then you'll keep your rising momentum from that larger air jump during transform and we'll be able to recover as Zelda from a higher angle.
I imagine you might be able to boost transform to avoid certain characters' followups because of the intangibility of transform, but only combos you could escape by jumping so maybe it's not as worth it? I've been harping on them to speed up the start up time of sheik's transform to facilitate transform as a dodge for her. I haven't asked a sheik main what they'd think of this and if this would make them more inclined to use it. What do you think? and what could they do to sheik's transform to make you want to use it?
Another thing about transform is that if you do it in the air, you can buffer any input besides a cstick one and sheik/zelda will cancel all endlag and do the input on frame 1 after the loading time of transform. So supposedly you could bait someone by transforming in the air and punish with a lightning kick or a slap. Imo this maneuver as a whole has such a long start up that it becomes completely impractical and is essentially just a flashy disrespect way to kill. It also has a a very simplistic way to be punished so imo it doesn't work how it was intended: none of zelda's aerials come out fast enough to avoid always being punished by your opponent once they learn to throw out a lingering hitbox (like falco's dair or link's nair) just before you come out of the loading sparkles.
Finally, a more theoretical advantage to switching to zelda mid battle is just that they have different playstyles and people say that a smart player can use this to trip up your opponent. I think this is a vanishing advantage however, because if we saw a lot of good shieldas, then people would just get used to it. Nevertheless, because they function differently, if you learn zelda then you can take advantages of situations that do favor zelda over sheik:
A sheik player once told me that sheik in general, but specifically with the sheik/fox match up, when the opponent is at low percentages sheik needs to play incredibly carefully, dashing around, needles, running in to grab and getting chain throws to build damage, but when fox reaches 55% suddenly your dash attack will pop him up for a combos and the match up suddenly turns in your favor. Zelda excels at combo-ing fast fallers at low and mid percents so you can be zelda to build up the percents pretty fast, throw or knock them away so you can transform and then death combo with sheik. People always say sheik is fast so you can rack up damage but zelda can rack up damage with less effort. LC nayru into an usmash is a sure thing all the way to high percents and you can almost always follow it up with a nair for a combined total of 39%. Her upthrow will CG spacies from 0-60% easily, you can cg a fast faller twice and easily followup with a bair for 39% as well. Against other characters her dthrow deals 15% damage by itself and sets up for a tech chase, so you can see what I mean. At higher percents, yes zelda has powerful kill moves, but they are so slow that they all require either a garaunteed set up (which she does NOT have) or a good read, while sheik on the other hand, can easily bait an opponent with her superior mobility, set up with a dtilt, bthrow, dair, fsmash, lc needles, or dthrow for a slap. Sheik's moves come out much faster than Zelda's, making it that much easier to punish an opponent on a reaction.
Anyways, this is all I can think about about sheik to zelda, hope this helps!
