If you can get the multivalve off, go ahead and change the sand. It's not worth trying to fix if it's gummed up. Sand is going for $5.99/50# bag, so 3 or 4 bags is $18-$24 and that's 150-200 lbs.
The only thing to worry about is be very, very careful around the lateral pipes, both getting the old out and the new in.
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