I have Simpex VT-531 speedlight flash, and currently using yongnuo RF-603 trigger. I like to upgrade to a new yongnuo HSS trigger. Is YN622N suitable for my Simpex flash?
The Simpex VT-531 is a manual control only flash with no TTL or HSS capability. It does not matter what trigger you use with it, you will not be able to use HSS with that flash.
Switching to the YN622N trigger will not increase the capability you already have using the Simpex VT-531 with the RF-603 trigger system.