Do you have to upgrade from 11 to 12 then to 13, No.  Since there is no upgrade path from 11 to 13 it will cost $249.95 (just looked at store price).  If you upgrade from 11 to 12 it is $99.95 and from 12 to 13 is also $99.95.  Doing the 2 upgrades will save you $50.  Plus you also have access to version 12 if for some reason you find it helpful

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