I don't have any experience with the Global Keratin brand, but had I only done 1 application of QOD myself, my hair would have (and did) reverted in no time. It wasn't really reversion that was the immediate problem, but that it really didn't relax my 4a/3c curls much. The first application result was so disappointing, I immediately started trying to figure out what I could do to improve the result, and I decided that it seemed the looser the curl you start with, the looser the end result, so why not apply that to myself and just keep applying the keratin over and over without waiting for the curl to revert, thereby starting with looser curls each time until the desired curl pattern was achieved. Eventually, my curls stopped reverting at all. I did 4 full-head applications in a 2-month period, and after that last application, my curls were about a loose 2a--barely even waves--too loose. So, I stopped all applications, and it finally reverted to a 2c/3a after a week or so, and there I left it, only doing root applications every 3-4 weeks. I've only done one more full-head application since then--about 5 weeks ago--and I did it with very few passes of the flat iron because my hair was still about a 3a/3b curl pattern at that point...it just seemed to be getting really fine and frizzy, but the curls were still relaxed, so I just wanted to do a full-head touchup. I did ~10 passes at the roots, though. My curls are about 2b right now. Roots are problematic because they are either too curly if I don't do them every 3-4 weeks, or they are too loose if I do.
The point to my post is that--at least with my hair--the results ARE definitely cumulative, esp if you do multiple applications at a higher frequency instead of waiting months in between--and eventually, the curl stops reverting at all. Again, this has been my personal experience with QOD Max and QOD Gold. This is why I DIY...this would be obscenely expensive and time-consuming in a salon. Also I have total control over how it's done.
I forgot to answer your question...I have had no obvious damage from my frequent BKTing. My hair has grown and grown and grown. So, I
would have them redo it, then I would consider DIY for future maintenance (unless you can afford to go to the salon often).