Well let's see
Venom pre cleansing :
You would get a straight update, but you would have to "educate" A child that constantly is angry. However it is possible with time to calm him down if you have a strong mind and will, as well as an healthy dose of kindness (in other if you are Alfred)
Post cleansing, straight upgrade with a stable symbiote that is basically innocent.
Grendel : welcome to the knull hive. Enjoy being a psychotic Uber powerful symbiote.
Klyntar hive pre cleansing
Your symbiote will actually urge you to be good and help people.
Post cleansing :
See Grendel but not as powerful