Myself I hold 8 black belts; I startsd Judo at the age of 6 and that was my first black belt ranking tben I went to some Korean styles and studied 1) Tang Soo Do 2) Tae Kwon Do 3) Hapkido. Korean technique's are all pretty similar so crossing over was fairly easy, just having to learn various technique's that weren't taught, this was more so in Hapkido than the other two.
After that I went to Isshinryu, Bujinryu, Shotokan. Again all very similar in technique and yet again I had to learn the various techniques I didn't know. Some weapons, katas, hand techniques, then gained my ranking in those.
Then my last one I only received my Brown Belt i was Kenpo, it wasn't all what I expected as I gained ranking so I moved on. I am current working on gaining my belt in Arnis through GM Andy Sanano.
If styles are very similar or I have someone come to me from another style and claim to have a Black Belt I do my research and if they do, then I will either one see what level they are with my style and promote them to that level or they start from the beginning.