First, nobody knows for sure.
Most likely the OTA will be followed by a leak of the official ics ruu executable file or zip file.
Those on the leak will use the ruu file, as long as the version number is higher, and it most likely will be.
HTC posted information about the OTA, and listed the latest leak version number. Then it was taken off from HTC's website. So it is possible that the OTA will indeed be the last ics leak.
I think the worst case is people on the leak will need to do s-off, then ruu to gingerbread, then optionally restore a nandroid backup of his gb system saved from before installing the older ics leak, then OTA to ice cream sandwich.
If it turns out to be something complicated, the devs will figure it out.
regards,
Howard