Hi,
In general ,the Release Characteristics for PO are decided during design stage.As you need to use only ONE Release Class for PO -if business needed further you can create New Release codes,Release groups and then design new release strategies.
Creating new Release Characteristics and adding to same ONE Release Class not advisable in running system but if needed- before changing existing Release Procedure and Release strategies in production system, Be sure that all old purchasing documents should be released with old release values with old release strategy and then carry out changes- but before doing
keep properly tested in Development system & Quality system and finally transport related transport request number related to your new release strategies to production system.
NOTE:If business needs,use Function Module - ME_REL_STRATEGIE_EKKO to define more than one class.
Regards,
Biju K