Not that I've ever added as much as 60 lbs. in a single go before, but IME, it's usually harder on a system to pull out rock than it is to add new rock in. At least I've found that I've spiked a tank by pulling out too much at once, but I've never spiked a tank by adding rock.
Anyhow, I would think you'll be OK if you use cured rock.
The only caveat I can think of is that 60lbs may be a fair amount of rock, which may involve moving things around, etc., which in turn may prove to be a slight disruption to the system. That said, it's not really any worse than a re-aquascaping project and usually things settle back down after a day or so.
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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