You can add or upgrade an exisiting solar system but there are considerations to be looked at before this can happen.
Firstly, how old is your existing solar system and does your current inverter have any unused capacity to handle additional solar panels?
Secondly, is there enough space on your roof to accommodate more solar panels?
Thirdly, does your current inverter have the required PV capacity to convert sunlight into energy?
There is also the option to add a complete new system alongside your existing solar system however, if your current solar system is relatively old, then replacing the whole system is the best option.