There is no reason to install hard seats for unleaded fuel although a lot of people think so.
One reason I would leave the rest of the springs alone is valves wear in to one spot and if you change the position of the valve in its seat, while changing the spring, it might not seal and end up burning.
If you mark the valve position to the seat on each valve you may be OK but...
If you change all of the springs I think you may as well have the valves reground just to be safe.
A eBay search shows a common
spring compressor. They can be rented also.
