Stu, is it possible, the humidity in your house decreased since winter? Maybe you had a humidifer running and turned it off when the days warmed up??
Also, check for insects. I hope I'm wrong, but look for scale, (lumpy brown armor-like bugs) mealy (cottony white patches, mites, (webbing) or any flying insects.

If possible, spray the trees daily w/a mister. This will help w/humidity..And check for insects..If you find any I or someone else will suggest cures.

Repotting wasn't the answer..in fact, this can cause more harm than not doing anything..How large of a pot did you transplant in? Check the diamter. Run a measuering tape across the top of the pot..

Be sure the soil doesn't stay wet..let it dry between waterings..this can cause wilting and yellow leaves. This is very important.

I'd hose the foliage, but try not getting soil wet..This may perk it up.

Too much fertilizer can cause salt buildup, but you'd have to really use a lot for this to happen..some ppl say it's best not to feed at all rather than get salt buildup, but I wouldn't go to that extreme..Citrus need fertilizer and iron. If one thinks they overdid it, all you have to do is flush the soil which basically means running water through the soil allowing it to drain out the holes of the pot.

Have you ever tried Superthrive Hormone? I love this stuff and it works like a charm..If you see it in your nursery or online, I'd advise buying a bottle.

Make sure your tree is getting good light, allow soil to dry out, and mist leaves daily..Check for pests. Toni

Well, I certainly hope your tree makes it, but I have to say that it makes me a little concerned that someone--especially me!!- could give you entirely well-intentioned opinions that would lead you to be gutted... :)

How did the roots look when you repotted--were they healthy or were they rotted? Sometimes, with all that dirt getting everywhere, it can be a little hard to tell by color ( pale being better, dark being rot); if the roots are firm they are probably ok; if they are soft and squishy (and usually darker), they're rotting. Also, your plant can have trouble and not all the roots will be bad, but just some of them. If they're dead and nasty be sure to at least remove those. I've read on how some people soak plants with root rot in a light bleach solution but I don't know anything else about that.

There are a lots of possibilities for why your plant declined, but I'd guess it is too much water retention in your potting soil... potting soils and other organic media will compact and degrade over (a rather short) time and lead to poor aeration/too much water retention at the roots, leading to root decay and an inablility of the tree to actually take up any water or nutrients that it needs through those rotted roots, hence the curling leaves.....so if we go with that theory, then changing the potting media would be a good thing...And, it would explain why you had not changed a thing in your routine and had trouble: the media broke down and couldn't handle your *normal* water schedule...

Of course, you also do not want to overwater--do let the soil dry fairly thoroughly between watering.

It would take the trees a little while to regenerate new roots and recover if all of my guessing and posturing were accurate...btw what are you using to grow in?

The other thing that may be an issue is the new sun exposure. Do you think they could be getting sunburnt, since they've been indoors all winter? The answer to that would be to move them into shade/dappled shade and gradaully increase their light exposure.

HTH.