Hmmmmmm. Yes, that certainly is an interesting article. Before everyone panics, though, consider this:
Call me a skeptic, but I get the impression that those authors are maybe just a tad too sensational with their report. I've seen the prior few case studies that they mention, where CC supposedly segued into UC, but in most of those cases, as these authors point out, farther down in their report, inadequate followup precludes solid substantiation that CC actually progressed into UC. Most likely, the CC had nothing to do with the fact that those few patients developed UC, it was mere coincidence. CC does not make anyone immune to developing any other IBD - we all have the same chance of developing it as anyone in the general population.
This case study is claimed to be thorough enough to avoid those prior problems, but the authors make so many radical claims that run contrary to general knowledge about both CC and UC, that it is difficult to take them seriously. They pull out all the stops to make CC look like one of the most fearsome diseases to ever come down the pike. Consider the facts of the case:
Note that this was a case study of a single patient, and note especially how drastically many of the claims of the authors deviate from conventional thinking about the diseases involved. For example, they lead into a discussion of their startling observations about their patient, by pointing out all these sensational claims, which run contrary to typical CC behavior:
Other complications include spontaneous peritonitis with free perforation of the colonic wall (9), submucosal dissection (10) and colonic ‘fracturing’, apparently during endoscopic instrumentation (11). Colon carcinoma may also complicate the clinical course of collagenous colitis (12,13), while lymphoma and carcinoid tumours have also been noted in some case studies (14–16). Recently, a paraneoplastic form of collagenous inflammatory disease involving the small intestine and the colon completely resolved following resection of an invasive colon carcinoma (17). The present report documents yet another serious colonic complication of collagenous colitis, namely, the development of severe and extensive ulcerative colitis.
The red emphasis is mine, of course. What's this "paraneoplastic form of collagenous inflammatory disease involving the small intestine and the colon"? That doesn't sound like CC. Furthermore, an invasive colon carcinoma, and CC, are generally considered to be either mutually exclusive or at least unrelated.
Over the next eight months, her symptoms became more severe despite treatment with 5-ASA, and the addition of oral prednisone (40 mg daily) and azathioprine (100 mg daily).
IOW, the patient didn't respond to mesalazine, prednisone, nor Imuran. Really? This is obviously not a typical patient. Right? They go to extra pains to point out that not only did all the thickened collagen bands in the lamina propria completely disappear, but lymphocytic infiltration totally disappeared, as well. Remember how it took Polly approximately 10 years on the diet to get normalization of her epithelial histology? Remember how no one else on this board has been able to have a follow up colonoscopy that produced biopsy samples with normal histology? Return to normal histology does not happen within a matter of a few months - it takes years. I suggest that when the patient in this case study developed UC, the disease was so aggressive, that most evidence of CC was obliterated, and any that might have remained, was simply overlooked by the researchers, in their dedicated attempt to produce a "sensational" case report. Even if what they report is absolutely true, obviously the patient involved was no ordinary CC patient - this would have had to have been an extremely unique case, since so many typical characteristics of CC and UC "went out the window", as the story unfolded.
While associations between collagenous colitis and other inflammatory bowel disorders (eg, Crohn’s disease) have been reported in the same patient (22,23) or family (24), this could occur due to chance alone.
It certainly could. Or, maybe that report is as honest and as accurate as the day is long.

Just remember, though, that the conclusions of most research reports are wrong. And that's a fact. The trick is in figuring out which ones are valid, and which ones are not.
Also, as we all know, there are exceptions, (somewhere), to every rule.
Tex