If you are a 410A refrigerant system, which contains equal parts of R-32, and R-125, and you have had refrigerant loss, technically what's left in the system should be recovered before a new and complete charge is added. This is because neither refrigerant parts will leak out at the same amount. One of the reasons is, molecule sizes are different between refrigerants. Where one can leak trhough a micro hole, another can not. Yes, micro leaks are a thing, a leak test should be done first to verify if and what size a leak might be. Any and every HVAC system can be opened, and accessed when it comes to Freon. R-134A is a non blend and can be topped off.