I changed my Super P/j 5 to E-C. I just stole the C string out of a six string long scale set. I buy single C strings from bassstringsonline.com, to complete any sets that don't offer high C strings. I did that for using five string sets on the six string, too.
Most C strings won't have any taper, so as long as they are long enough, and balance with the other strings, it's fine. D'Adarrio xl and DR sunbeams are my usual pick for C's.
I would just shift a five string set over and try with a single C.
At the nut, I have the tiniest rattle on the open D string. Not a deal breaker. A little intonation work at the initial set up and everything is fine. I raised the pickups a tad as well.
BSO.com has quite a few long scale sets for six strings, and you can buy individual strings for many sets. Since the low B is the big money, it's worth it. C strings are usually under $5. They have Dingwall sets as well. You'll have a couple extra B's for the switch back to standard.
Payson has a number of nickel round sets for Dingwalls. I really liked the reduced core with a D'Adarrio xl C.
It's super fun to have the higher voice for chords and solo lines. I don't really miss the low B, and it's much easier to mute the five string. I still miss that AB 1 six string, but I'm really digging the Super PJ strung this way. I should have done it years ago!
I figure I'll go back and forth as the mood strikes, since I've managed to get by without doing any work on the nut.
I figure I will leave my AB1 in standard 5 string tuning, and my Combustion as well. See how I like my Z1 E-C if I can figure out strings.