Hi k2A

You can Migrate your Process Instance version using one of below option.

•  Use "Process version migration utility" (can be downloaded from K2 Market)  to migrate running instances of processes to a different version of the process definition.

        Or

• Change the default process version  to old version  using K2 workspace  and reacreate the  new instance for process instance you want. 

          Change process version using workspace

           Start new process instance using workspace 

(once  you will change the default Process defination it is used as the master process from which each process instance is generated).