Changing with fish in tank may be possible if you use the technique I read here once before....from laurleebp maybe.
Clear a small section of the tank...then use a tube like a gravel vac tube without the top on and slide the new substrate down the tube. Do a small section at a time. Biggest challenge may be dealing with the cloud of detritus that is sure to explode out f each section of substrate as you remove it.
May not seem it, but removing everything into buckets may be less stressful for all.
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