As long as you have a version of the Download Manager and the Serial number you should be good forever. You can also just copy the files downloaded on your first computer and move to another (that way you don't have to wait for the download). The files downloaded replace the old DVD and would be the same as if you got the DVD.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13