AIFF is lossless, and it should be a "bit perfect" copy. therefore there shouldn't be any difference between JRiver rips into (FLAC or WAV) and itunes rips (into AIFF)
that said, i don't know how iTunes actually rips and if it "colors" the copy or not...lossless unfortunately doesn't always mean bit perfect, it depends on the software.
honestly, if YOU notice a difference, then re-ripping might be your best option. personally i would do some ABX testing (in foobar for example) and see if i could actually tell the formats apart in a blind setting. if i found any difference under proper testing, i would not hesitate to re-rip...if i couldn't tell i wouldn't bother.