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John , technically you could do it and still obtain a large enough 100% and 75% illuminated field.
But you would need an ultra low profile focuser and a change in the mounting of both the diagonal and focuser to accommodate the change in diagonal.
As it is on a standard scope I would assume you have a rack and pinion focuser and about a 300mm diameter tube your 100 % fully illuminated field would be some ware from 4-8mm in size with a 50mm diagonal.
This would be O.K. for a ToUcam and short focal length eyepieces but no good for long focal EP's---eg deep sky.
There are a few variables that will change this and I am just guessing on those sizes.
A Newtonian optimised for a 50mm diagonal will give about a 10-15 mm fully illuminated field which would be enough to illuminate most CCd chips and eyepieces.
The 75% field will illuminate any low power eyepieces.
The thing is the reduction from your current 25% obstruction down to a 20% with the 50mm is not worth the expense or hassle .
You would most probably not notice the contrast increase of 5%.
A planetary optimised Newtonian has to be designed from the start to get all distances correct to get the most from a smaller diagonal.
I have a planetary 12.5" f6 Newtonian with a 16% obstruction ratio and 20mm diameter 100% F.I.F. but could have gone smaller at the expense of field size but it is really not worth it that much once you go under 20% , the differences are not noticeable.
Just be happy with the fact that your 25% is way better than a 35-40% of some SC scopes and if your after a performance boost for planetary work may be look at a better quality diagonal if need be and make sure the tube is well blackened inside , also that the tube is long enough past the focuser .
Hope this helps?
Mark
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