If you're intersted in having the JW mod 460's and ready to spend the money ($175) to have the second body mod'd, it's a pretty good deal price wise. JW mod is $175/cap, a pair of stock 460's runs about $350-400 in good condition, and the ck61 caps are about $225 a piece new.
Have you listened to any JW mod 460 recordings? IMO, the JW mod 460's are considerably better than the 391's & C4's (I had C4's before I upgraded to these). The only drawback would be that you lose the ability to run actives like the 391's since the extension cables & ck_x cables are VERY hard to come by. Is your Dmod UA-5 an Oade mod? If it is, you could upgrade to the T+mod eventually, and have a GREAT sounding rig IMO. KCMoeJoe has a lot of recordings up on the Archive that you can stream using JW Mod 460's > Pmod/Tmod UA-5.
The key thing on the rolloff switches is that there is one vertical pad. There was an early version that had 3 horizontal pads that JW doesn't recommend mod'ing. Since one of them is already mod'd, and they both have the same switch, I'm assuming that they both have the single vertical pad.
I don't think there will be any issue with buying them and having the second body mod'd. I'd call JW and make sure that he doesn't foresee a problem. Don't bother emailing him. His number can be found on his
web page. I'm pretty sure it's his cell, or that he has his office phone forwarded to his cell when out. I'd also ask him about sending in the body that already has the JW mod just for a checkup. He may say this isn't needed, but it's worth asking.
You can find a summary of the JW mod 460's about half way down the page
here that I compiled from various threads/websites.
I'd be happy to try and help answer any other specific questions you've got.