Going to 75% of the original size means you won't have much issue with most of the design changes. Try to shoot for the next smaller wood size in as many cases as you can but don't go to less than what is typical for the size of model that the design will come out to be. For example parts made from 1/16 will need to stay 1/16 but some 3/16 parts could be reduced to 1/8 where they are not flying surfaces. But if the horizontal and vertical tails on the full sized were made from 3/16 solid sheet then consider making them built up instead on the smaller version with 3/16 x 3/4 front and rear and 3/16 x 1/8 ribs. This will lighten the tail with little reduction in strength.

The new smaller model will need to be a lot less than 95% of the weight of the full sized to fly well. Part of this is that the wing area goes down by the square of the size ratio. So your .75 scale model will only have .75^2 = .56 of the wing area. On top of that the smaller model will require a lighter wing loading than the 40 version. So with both the area ratio in mind along with the lower wing loading number you'll be looking to make the new model be no more than 1/2 the weight of the 40 sized version for it to fly well and slow down well for landings.