Can a relay for 5.5V be replaced with 3.3V relay on the module?
The coil current  will likely be about 50% higher for the 3.3V relay compared to the 5V, and the supply voltage is less, so it would be prudent to look at reducing the value of the resistor from the optocoupler output to the base of the drive transistor. The optocoupler LED series resistor maybe should also be reduced. 

You will need to supply a schematic with part numbers and values for us to make more specific recommendations. 



You've added a schematic, but the part values are not readable, also I suspect maybe some parts are not installed as there appears to be two options (with and without isolation).