Your ship will be disabled and your shields will be down while the conversion takes place.
The duration of the conversion process depends on the amount of energy being converted into shielding.
The Shell Shield increases Shield Capacity by 926 and increases Shield Regen by 5. No other primary shield comes close to the capacity, but even the basic shield offers better regen.
The picture shows what it looks like when Recharge Shields is used. Normally, the Shell Shield looks like any other.
You cannot use this ability unless your shields are not at their full capacity.