actually there are several factors (I'm a licenced insurance agent here in Oregon
insurance premium is calculated through symbols. a symbol is a numerical value from one to... well I dunno but I've seen as high as 35. These symbols are based off of theft rates, how much car parts cost for that vehicle, how much damage the vehicle is capable of to others, etc...
for every coverage you have (BI liability, PD liability, UMPD, UM, COMP, COLL, PIP [or maybe med pay depending on the state] there is a symbol based off of the vehicle)
a civic may look like this
BI: 8
PD: 7
and so on
those numbers translate into premium which is mathematically offset lower or higher using a formula with variables found in your driving history, the amount paid out in your prior claims, your age, and sometimes even gender and credit score. Hell you even catch a break if you're married or have kids... but I digress...
that being said you have to look at the vehicle, Hyundai parts cost more then honda parts. a 4X4 1 ton will do much higher damage then your average civic.
you have to assume that a much higher amount of personal injury during a collision is capable in a convertible, BUT, the Accord is famous for being the most often stolen car in America.
I would expect a slight increase, nothing crazy unless your record sucks...