My logic behind that is "annual" navigation updates contain the data for the maps (including 3D representations of some parts of some cities) as well as gracenote database updates. The brains behind how that data is shown and the interface is in the firmware/microcode/whatever-you-want-to-call it that is unique to a generation of Nav (2013 RX has Gen 7), but the Lexus Navigation site does not yet show the 2014 models, so perhaps there is a Gen 8 you are referring to with the 2014 IS that has additional capabilities. I've not seen a new IS and don't know.

2013 RX has/had the latest gen Nav as an upgrade to what the 2010-2012 RXs had, and is actually somewhat of an improvement in graphics quality and the interface from what those vehicles had with Gen 6 Nav. I personally do not believe it's possible, or I'll say "realistic" to hedge my bets with some future poster that wants to correct me and void their warranty and spend a lot of money effectively trying to mod their vehicle with a new head unit and/or some hack of firmware. Lexus tests the firmware that runs the Nav as part of the whole vehicle -- they are simply not going to sell you or provide to someone the code from a different vehicle that does not match your VIN. Only Lexus knows to what degree a particular set of their hardware would work with a different set of their firmware. More importantly though, who knows what side effects there could be to a mix-and-match combination given how the Nav is so integrated with other vehicle systems.

My suggestion is if you don't like the Nav on the 2013, wait a little while (speculation on when that is is in other threads) and check out a 2014 RX when you can get your hands on one. There is no telling FOR SURE if an update to the Nav generation may happen, although my money would be on "it's doubtful" given Lexus just did a mid-life update to the RX last year.