Even if you do accidentally delete the domain from the Manage Domains screen, you can just add it back, with no harm done. So relax – nothing you do to a parked domain from Manage Domains will be destructive in any kind of permanent way! (the Registrations screen, on the other hand…)
“Deactivate parked” is the same as clicking Edit, and choosing DNS Only. So you can either click Edit, and choose Fully Hosted (or one of the other available options), or you can click Delete and then re-add it.