Well, I'll play devil's advocate and offer the opposite view. I've had a 4-wheeler with a transfer case with a full-time option in the past and my current XJ has the 231. I don't think the benefits of full-time 4WD are worth the effort to make the swap, even if you do already have the t-case sitting there.
With shift-on-the-fly you can put your 231 into 4WD anytime you need it, and then take it out when you don't. The only thing a 242 t-case gives you is the option of saving the teeny-tiny bit of effort that's involved in shifting into and out of 4WD.
Now, some people will say, "but you'd have to be constantly shifting back and forth in some situations," to which I say, nonsense. I live in Colorado, drive up into the mountains regularly, see ice and snow all the time, and have yet to see conditions where I needed to be constantly shifting into and out of 4WD. What I see are conditions of two types: 1) It's pretty bad and you shift into 4WD and leave it there; the few small patches of dry road are not enough to cause any problems when in part-time 4WD. 2) It's not so bad and you only need to shift into 4WD very infrequently; maybe sometimes when you start off, and once in a while to give you more control going around a corner.
Another thing some people will say is, "with full-time it doesn't matter if it's icy or not, I just put it into FT and go without even caring!" Now, those are the people to WATCH OUT FOR!!! You can't drive like the road is dry, just because you have it in FT-4WD. You still need to be aware and careful when it's slippery, because your Jeep in FT-4WD will not stop any better than any other car on the road, and it won't corner any better than any front-wheel drive car out there (like my Toyota Corolla, which does very nearly as well on snowy roads as my Jeep).
If you really want to be lazy then do the swap. You've got the t-case already anyway. I wouldn't bother, though. I'd just keep the other t-case as a spare.