Barry8355

Update on this topic:
I bought 2 x US3000C's
Charged the bank of 3 old  US3000's to 100%
Whilst the US3000's were charging I connected the two US3000C's together to balance them.
Disconnected the 3 x US3000's, connected the 2 x US3000C's. Turned system on and programmed the Goodwe to accept 2 x US3000C's
Whilst the 2 x US3000C's were charging (they arrived at 52% SOC) I contacted Goodwe support and they updated the firmware on the inverter online.
Once the 2 x US3000C's were at 100% SOC I joined all five batteries - US3000C master, US3000C, US3000, US3000, US3000
Turned the system on and programmed the Goodwe to accept US3000Cx4+
All well and system running 100%. I did not update the firmware on the old 3 x US3000's
Hope this is helpful. 

Having now established that I can just add a couple of UP5000's to the battery bank I am no longer selling the 3 x US3000's. Thanks to all who gave advice.
According to Pylontech support so long as the most recent battery is the master all will work fine. Evidently all pylontech are compatible if configured correctly
Only advice I am sceptical about was to put one UP5000 as master, then the three US3000 and then second UP5000 last!!!!
 UP5000>US3000C>US2000C>US3000>US2000
For same type of module always use the latest production unit as master.
*Mixture using battery deployment option:
Master battery (1st) UP5000
Slave 2nd ~8th UP5000/US3000C/US2000C/US3000/US2000
Slave 9th ~16th UP5000/US3000C/US2000C
(Copied from a post by SYC Oct 2021)