Well, you're gambling by not changing the belt. I'm pretty sure that the SC engine is an interference engine, meaning that if the belt breaks there's an excellent chance that internal damage will happen. (This is not the case with non-interfence engines.) You have heard , and will hear, of owners who have gone beyond the 90,000 miles or 9 years that Lexus sets as limits, and they have had no timing-belt-related issues. As i mentioned, it's a matter of taking a chance.
The issue with the 9-year parameter is the the belt, being a kind of rubber, may deteriorate. I've done the job on my car and it's pretty straightforward. Using a kit from Toyolexyus I thus used "official" parts and spent about $350.00, as i recall.
Jim
incidentally, all of my parts were in excellent condition but I enjoyed doing the job and I have peace of mind.