The stock offset is 45mm. If you only change to an 18" front wheel you're lowering the front end .5", no need to worry much about changing geometry. However if you then raise the rear with shocks longer than say 1" as well then that will surely reduce trail enough to potentially bring on a tank slapper if something upsets the chassis. I'm preparing a set of NSR125 35mm triple clamps for a fellow here for a project he is planning.
Count on 25mm up or down on a 1422mm wheelbase to result in a 1 degree change in rake. As an example on a stock frame mine was lower in the front by 12.7mm, the rear shocks were 44mm longer. This yielded a rake angle of approximately 24 degrees (stock=26d) and with 45mm offset reduced trail to approximately 89mm. (stock 105mm) Dangerous numbers, and it eventually caught me up! Now my rake is reduced 1 degree further to 23 degrees and with 30mm offset clamps my trail figures are 99mm, still quick steering but with plenty of trail. Just so you have some figures to work with.
Honestly though these type reactions aren't a guarantee, I raced three years this way and never felt a twitch....it did lead to disaster eventually, but on the street you might never have an issue. If you want to be sure it never happens, then certainly make the change.
