Storm and Ascendancy are two decks that lend themselves quite well to transformations, and also benefit the most for the reasons you mention. Young Pyromancer is a definite inclusion, but I’m not sold on Thing in the Ice. Yes it’s pretty easy to flip but it’s also more vulnerable than Pyromancer and do you really need it against creature decks? It seems like just racing is your best call in those matchups. I’d transform with Pyromancer, Snapcaster Mage and possibly even Delver before Thing.

Gifts Ungiven, in every format it’s been a thing in, has lent itself to what I call a “progressive transformational sideboard”. What I mean by that is that the nature of the namesake card means the deck has the capability to transform to varying degrees–a well constructed sideboard can allow you the freedom within a single match to alter your gifts piles, double down on a combo gameplan, or to cut gifts and its support cards entirely in favor of a more midrange gameplan. Not recently, but a year and a half ago or so we were seeing some 4c gifts builds on stream that had a sideboard plan on the play of doubling down on reanimating iona or elesh norn or something, and a sideboard plan on the draw of cutting gifts entirely in favor of a straight up midrange-style game plan. It’s not common to see, but most formats at one time or another have decks for which this is possible.

In Vintage, you had a period of time where TES decks could sidestep into an awkward version of shops to completely dodge the hate that drain decks would bring in. In legacy, you have the RG lands deck today, which can double down on being a combo deck with some backup primeval titans, or it can go the full prison route with sphere of resistance and chalice of the void/trinisphere, sometimes within the same matchup. In old extended, you had the thopter-depths decks that could sidestep the hate by switching to value gifts piles and removing their reliance on the graveyard entirely in games 2 and 3. In a similar vein, you had the all in red lists that could board into something of a prison shell.

In modern, we have the aforementioned gifts lists, and early on in the format we had UR twin decks playing one or two bolts and 2 or 3 of relevant combo pieces, with sideboards that could range from moving all in on the combo to cutting it almost entirely.

In standard, we’ve had the old bolt-warp decks that could transform into a value-based midrangy burn deck or even cut the pyromancer ascension entirely in favor of some creatures, rendering them an awkward but effective tempo deck with time warp effects to play once their threats were online. More recently, we had the mono blue devotion decks, that could end up anywhere on the spectrum from a dedicated tempo deck against UW elixir, to attempts to straight up beat them at the control game.

This general piece of sideboarding strategy, having a fluid way to either transform entirely along a spectrum with respect to ONE OR TWO COMMON MATCHUPS is a trait that I think many experienced players miss–if a matchup is common, you can snipe a lot of percentage points by having a variety of ways to sideboard for the matchup. The most recent one I can think of? UR twin mirrors just before twin got banned–the assumption was that players would be more controlling and light on the combo in sideboarded games, but we all know there were times on camera where one player took a gamble and went all in on the combo in sideboard games, turning the matchup into a shell game to either devastating or comical effect.

I myself am considering to include transformational sideboard just because most of the decks I play are combo ones and they fall miserably against said hate and it sucks horribly to lose games that way. Maybe add some Things in The Ice’s and Young Pyros into my Storm and Jeskai Ascendancy board lists?