Wow, recasting in 3e? I'm shudder to imagine all the number crunching, but how did you do it?
Organically. Initially, I used the same stats.
Then you notice that for support creatures, you really need only a few abilities, ideally ones that help their allies even when seriously outlevelled - so you'd simplify their spell list and make three bullet points do A then B then C (in 3e this would be stuff like flanking, spells with no save, buffs, aid another, or game-changers that may almost never work, but even if they do only 5% of the time pose a risk). By doing that, you can get creatures that remain threatening even to PC's 10 level higher than they are.
Then you notice things die in one attack, so I (basically) made minions - no rolled damage, any damage over a threshold kills em, and a variant of iterated attack to discourage completely ignoring them.
But yeah, there was a lot of number crunching, which is much less work in 4e. I wouldn't exaggerate though; you get proficient with it, and you reuse stuff, so it's not like you're spending hours each session either.
I think the trickiest part is actually making the recast monsters feel the same. I tried to do that by explaining mechanical changes as intentional changes in tactics; just recasting a solo as a minion without explanation comes across as jarring to me. I'd definitely encourage thinking about an explanation for the mechanical changes, it makes the whole campaign feel so much more real if there's a reason the old solo can suddenly hit so much higher AC's and lost his aura, millions of attacks etc. And that's really campaign dependant.