A Merry Christmas to you!
1-Are the Christmas lights resistant to moisture? Of course, the heat of the LED is not high, but moisture can enter various parts. Since all lamps are similar, this is a reaction to the environment and moisture, so it is better to replace the set with a waterproof and insulated circuit.
2-Make sure the power supply circuit delivers the correct voltage and amperage to the circuit and no more, especially if you bought the power circuit separately .
3-Each LED has an internal resistance that reduces the current(V=IR). Replacing a string of lights with the other models probably is not a problem, but replacing two strings of lights with other models may leave too much current in the circuit due to the difference in internal resistance that will burn the rest of the LEDs.( upper limit for LED's is 20mA )
4-Adding a string more than the initial number to the group can reduce the current and be useful. I hope you also consider other environmental conditions.