a reconstructive transformation
Some structural changes are gentle rearrangements; a reconstructive transformation is not. It is the demolition-and-rebuild kind: to get from the old structure to the new one, chemical bonds must actually be BROKEN, atoms must migrate (diffuse) to new places, and the crystal is effectively re-assembled from the ground up. Think of dismantling a brick wall and rebuilding it in a different bond pattern — you cannot get there by just leaning the bricks; you have to pick them up and re-lay them.
Because primary bonds are severed and atoms must travel, reconstructive transformations need a lot of activation energy and time; they are sluggish, strongly temperature-dependent, and can be frozen out entirely if you cool fast enough. Graphite to diamond is the extreme case: carbon's flat sheets of sp2 bonds must be torn apart and re-knitted into the 3D sp3 tetrahedral diamond net, which is why diamond, though only metastable at room conditions, survives essentially forever — the barrier to reconstruct back to graphite is enormous. The silica transitions between quartz, tridymite and cristobalite are reconstructive too: the Si-O bonds must break for the tetrahedral framework to re-connect in a new topology.
The defining contrast is with a displacive transformation, where NO bonds break — atoms only shift slightly. Reconstructive changes rearrange the connectivity (the network of who is bonded to whom); displacive ones keep the connectivity and just distort it. Because reconstructive transformations are slow, they let us keep materials in structures that are not the equilibrium one at room temperature — metastable diamond, quenched high-temperature phases, glassy states — which is a large part of why materials processing works at all.
Diamond is only metastable at room temperature and pressure — thermodynamically graphite is the stable form of carbon. But turning diamond back into graphite means breaking the entire sp3 tetrahedral network and rearranging every carbon atom, an enormous barrier, so diamond survives essentially forever. Making diamond from graphite the other way needs high temperature and pressure (around 1500 degrees C and tens of thousands of atmospheres) plus a catalyst — a vivid measure of how sluggish a reconstructive transformation is.
A reconstructive transformation: primary bonds break and atoms diffuse to re-sort — slow, high activation energy (as in graphite to diamond).
The dividing line between reconstructive and displacive is whether bonds break: a reconstructive transformation must sever primary bonds and let atoms diffuse, which is exactly why it is slow, needs high activation energy, and why metastable structures like diamond can survive for ages at room temperature.