I was looking at the CIJ 62 custom reissue
That should be a very nice guitar.
Not sure how much money you want to sink into this, but...
You'll need a new pickguard, or modify the existing one.
If you are up for it (and are technically inclined) I'd say try the route and put in a humbucker. If you are not sure pay a luthier to do it. If it is done correctly and you are not happy with it you could almost certainly reinstall a single coil in the neck and put the old pickguard back on.
I've always liked the look of a Tele with a humbucker in the neck.
On a side note:
What type of humbucker are you planning to install?
Are you planning to keep the bridge pickup that is already installed?
You may not be as happy as you might think with the outcome without finding a good balance between the pickup outputs and wiring.
I would keep the bridge pickup (the pick ups are Texas Specials) I will put maybe some type of PAF humbucker or a 57 classic or Seymour 59
That should not be too bad, but the 57 Classic is a warmer sounding pickup and might be too dark with the standard 250k pots. You might consider changing the pots to 500k and putting a different cap on the bridge.
I'm not sure about the phase switch and pre-amp. That is out of my league. (Well actually all of this is out of my league) My responses so far have been based on some research I did when i was thinking about adding a humbucker to the neck of a Tele. I never actually took the plunge.
Anyways, changing the pots would merely be a better match for the humbucker. I think that 250k and a neck humbucker is too dark unless you are playing jazz exclusively. Now, a 500k pot should fix the neck pickup issue, but could make the bridge pickup too bright or shrill. A treble bleed cap on the bridge pickup would even things out.