Thought I would share the response I got from Universal Guest Services by email:
Me: I am attending Halloween Horror Nights and have a question regarding ticket upgrades. Is it possible to purchase the Rush of Fear pass and, if I decide to attend more dates, upgrade to the Frequent Fear pass?
UO: Please be assured you may upgrade your valid park admission ticket at any of our Guest Services locations inside Universal Studios Florida® or Universal’s Islands of Adventure®. Please be aware, the value of your ticket, not necessarily the amount paid, will be applied toward your upgrade. Ticket upgrades must be completed before the end of the day on the final day the ticket is used. Additionally, tickets cannot be upgraded for use by another person or for use during a separately ticketed event, such as Halloween Horror Nights® or Rock the Universe®.
The part I underlined is vague enough to make me concerned. I think I'll just get Frequent Fear and work with those dates.