If a 10% sucrose solution within a semipermeable sac causes the fluid volume in the sac to increase a given amount when the sac is immersed in water, what would be the effect of replacing the sac solution with a 20% sucrose solution

Osmosis is the net movement of water molecules through a semi-permeable memberane from a region of higher water potential to a region of lower water potential.

so in the first case, since 10% of sucrose solution has a lower water potential than the pure water, so water molecules will flow into the sac, causing the sac to increase in volume. note that sucrose molecules can't diffuse out since there's a semi-permeable membrane.

in the question, I'm not 100% sure what the "sac solution" is meant by, but I guess it's the solution inside the sac

so here if the sac sucrose is 20%, the concentration of water is larger in difference than the first time, so osmosis rate will increase.

but if the "sac solution" is meant by replacing the water of the first case, then the sac inside would have a higher water potential, as the sucrose concentration is more diluted. then the water molecules will flow from the sac back to the beaker or whatever container it is.