Convert is not quite the right word. Gasoline is a blended mixture of several major components and lessor amounts of certain additives. See yarddog’s post 25 for a description of this.
As far as refining capacity, the major components are probably fine as-is capacity wise for the switch to a single high octane grade. The lessor components such as alkylate could require capacity expansions. Refinery process pros would know better than I on this.
One driver for this is that the trend in engines is to lower displacement, supercharged and turbocharged engines operating at higher compressions that require the higher octane to generate their maximum HP/torque and fuel economy. A second driver is to simplify the tangled mess of octane blends, summer/winter blends and regional blends that create a logistical hell for refiners and distribution and in turn increases cost to the consumer.
This is outofsalt with the nice info in post 25.