Hello chenchen, You can leave the current cluster and just change the current type from 5.5 to 6.7. Please let us know if you have any other questions.
Hi Right now we have cluster of esxi 5.5. We plan to upgrade all to 6.7 soon. Same hardware (servers). What would be the best approach of settings in DSM? Should i create a new cluster, or can i leave the current one and just change the current type from 5.5 to 6.7 ?  
Esxi upgrades