You have a 68mm BB30 shell with two 30mm ID bearings in it and you want to know if you can just pop the NDS bearing out and install a 28mm ID bearing in its place?
Short answer is no, it would be a massive PITA.
Long answer. Looks like the Praxis BB uses outboard bearings, so you'd need to check three things
A suitable 28mm ID bearing is actually widely available and economically acquirable and the tooling to install it is also available and economically acquirable
The crank doesn't care if the bearings are circa 10mm further inboard, i.e. the bearing flats are sympathetic to 68mm rather than 86mm that the crank seems to be made for. This strikes me as highly unlikely given it is tapered like a GXP crank and expects the NDS bearing to be in the 'right' position.
you could get the spacers required to take up the slop that you'd know have on both ends of the spindle.
Personally I'd just pony up for the Praxis BB or get a proper BB30 crank.
Ok thanks, it's worth a shot - funnily enough Praxis don't make an EBB and I have an expensive Praxis crankset that I want use fixed and occasionally geared, with rear thru axle.
I'm not sure how the eccentric thing is relevant here - unless you are saying you have a BB30 shell in to which some kind of eccentric capability adding adapter has been added? But this would most likely limit you to 24mm spindle cranks anyway. Some photos of the set up might help.
You have a 68mm BB30 shell with two 30mm ID bearings in it and you want to know if you can just pop the NDS bearing out and install a 28mm ID bearing in its place?
Short answer is no, it would be a massive PITA.
Long answer. Looks like the Praxis BB uses outboard bearings, so you'd need to check three things
Personally I'd just pony up for the Praxis BB or get a proper BB30 crank.