There are some good reasons to go from a grid tied to an off grid system. Some people want the independence. Others realize that a loss of grid power will also cause their homes to lose power. Your reason might be different from these. Whatever the case, there is a right way to do it. This means having the right equipment and using the right process.
Here is the extra equipment you will need
You will need a hybrid inverter
A hybrid inverter is a grid forming inverter which means it can produce AC power without a signal. This is unlike a grid tie inverter which needs an AC signal to produce power. A hybrid inverter can also synchronize its phase with the grid power so it can tie to the grid. A hybrid inverter also has an AC transfer switch that lets it disconnect from the grid if the grid goes down.
If you have either a grid tie inverter or string inverters, you will need a grid forming inverter. This will allow them to produce power independent of the grid. You will also need an AC transfer switch to isolate them from the grid. This will stop them from sending power to the grid when the grid is down.
If you use a separate AC transfer switch, you will still need a grid forming inverter. Now, an AC coupled battery can act like a grid-forming inverter. But you cannot charge an AC coupled battery while it is working as an inverter. So you will still need a hybrid inverter if you want to keep the AC coupled battery charged when isolated from the grid.
In any case, you will need a hybrid inverter to convert from a grid tied to an off grid system. Being able to operate isolated from the grid is crucial to being independent of the grid. The total power of the inverters must be enough to power your entire home.
You will need batteries
When you go off grid, you will need batteries to store energy for when the sun is not shining. If you are off grid, you should have enough battery storage to last a few days. Also, batteries have limits on their discharge rates. You need enough batteries so that the total discharge rate can cover the household loads. The batteries can be DC coupled or AC coupled, but DC coupled batteries are more efficient. AC coupled batteries contribute to the total inverter output which helps.
Transitioning from grid-tied to off-grid
The easiest way to transition from grid tied to off grid is to transition through a hybrid system. You can start by adding the hybrid inverter in parallel with the grid-tied inverter to the critical loads. The hybrid inverter will be connected to the grid through the AC transfer switch. The DC output of the solar panels will feed into the solar inputs of both the hybrid inverter and the grid-tied inverters. The solar inputs of the hybrid and the grid-tied inverters must be compatible. The inverters also need fuses or circuit breakers to protect them.
The hybrid inverter will also connect to DC coupled batteries. That way, the hybrid inverter can charge the batteries in an energy surplus. The batteries will provide energy in a deficit. You should have enough batteries so that their combined discharge rate is enough to power the loads. Eventually, you will need enough batteries to last through at least 3 days. The grid tie inverters cannot connect to regular batteries. So you can use AC coupled batteries to store excess power from the grid-tie inverters.
The hybrid inverter can disconnect from the grid using its AC transfer switch. It will also provide the signal for the grid tie inverters to produce power. The critical loads panel is not connected to the grid, and it will be the start of your off grid system. Over time you will transfer more circuits to the critical loads until the entire house is on the critical loads. After that, make sure that you have enough solar panels, battery storage, and inverter power. If you do, then you no longer need the grid. You can disconnect from it.