You have to unwind and rewind them. Not practical.
They used to be bought in pairs and installed side by side, so that if one blew you could in a couple of hours wire up the spare. Then you can take the bad one out and either fix or order another made.
The problem with transformers are that they are made like suits. They are made according to the part of the grid they have been designed to service.
The only factors that build transformers big enough for our grid are made in Japan and Korea. We only make the smaller ones now.
Transformers do more than step up and step down voltages. They also change the frequency in order to decrease line losses and reduce the heat (another form of loss).
These guys are worried about bullets. You CAN fix windings if you absolutely have to. With EMP, the iron core around which the copper windings are made melt down. Then there is no repair possible.
EMP will also pop holes in the insulators and insulation at various and sundry places in the distribution network, which means you’ve got to deal with the leaks or they will either kill folks and start fires.
You can’t place an emergency order for these like you’d go to Costco to buy a big package of D batteries. That’s why this is a big deal.
You say EMP, by which I take you mean human created?
What about a massive solar flare? Could the end result of one or even a freak storm of several (say a temporary but powerful magnetic anomaly within the Sun) be as catastrophic as a man-made EMP?
Another worry wart! I have be assured by other FReepers that there is no such thing as EMP. That EMP is a booger bear of tinfoil hat crowd. (sarc)
They can't change the frequency, and if they did, it would lead to all sorts of interesting effects ;-) What transformers can do (and in fact, are physcially destined to do) is change the phase angle relationship between incoming power and outgoing power - something called "power factor."