Depending on the cartridges you are loading, you will need what Dillon calls a Caliber Conversion Kit (Shellplate, powder funnel, and locator buttons; $45) If you have cases which can use the same shell plate, no additional conversion kits are needed, but you may need to buy an appropriate sized powder funnel.
The 550 has standard threading for all your current dies.
Ifyou wish to make "quick changeovers" you will need the Quick Changeover Kit (Toolhead, Powder Measure, Powder Die, Toolhead Stand, $91). All your dies and the powder funnel ae on the toolhead and simply change the preset toolhead. This can get $pendy.
If you want to spend less, you can use the same toolhead, and screw the dies in and out and switch those components from the conversion kit. There ae also powder bar slides which are around $25-$30, which can be set for a particular powder weight and quickly swapped out in the powder measure.