switching between financing/cash is no big deal with the caveat that there is no additional discount for a finance deal. shouldn't be a problem, just call in to let them know. you will likely have a few bucks less due to no PPSA charge to register the lien.
as for paying off early, go right ahead. no car loans are "front loaded" or whatever. interest is calculated normally, based on the principal amount, so it seems like more interest at the beginning of the loan, but it is just the math of it.
if you are paying it off early, no big deal.. do it however/whenever you like, but as a courtesy, if you can wait a few months, the dealer would appreciate it. the reserve (like 100-200 bucks) that the dealership makes for sending the bank the business gets charged back if it's paid off too soon. don't let that sway you one way or another as it's not your problem, but if you happen to be looking at paying it off in 2 and a half months... maybe just give it another week or two if you don't mind.