"For starters, it'd probably need somewhat heavier shielding to handle the higher background radiation for flights beyond the van Allen belts. You'd also need a better heat shield to withstand the much higher reentry speeds. The Apollo missions hit the atmosphere at roughly 25,000 MPH when they returned from the moon, compared to about 17,500 MPH for reentry from orbit. Ok, you might end up with a 2 man Soyuz instead of 3. Sounds like a good start. "

Since high energy particles create secondary radiation, heavier sheilding might add to total radiation dose. Of course you could solve this by using sheilding which is has high amount of Hydrogen atoms. It might be possible to reduce radiation from high energy particles by using less sheilding. Also possible to use some kind of shelter- which could also be used for solar flare events.