Welcome to the forum from North Carolina. Happy you decided to become a member of the forum. There never are any stupid questions here. Just knowledge that hasn’t been transferred yet. I too like whitewalls. Have them on my 900 Classic. Here is some information on using the tire size you mentioned. You can use it and use your same rim.

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
140/90-16 5.0in 13.0in 25.9in 81.4in 778 0.0%
150/80-16 4.7in 12.7in 25.4in 79.9in 792 -1.8%

As you can see, the 150 rear tire is 10mm wider at the cross section, and 1/2" shorter in diameter. It'll make your speedo nearly 2% more optimistic, but that's a pretty safe margin of error.

Tire sizing numbers are explained thusly: aaa/bb-cc or aaa/bbRcc

a=section width. This is the actual width of the tire's carcass if you cut it in half and measured it edge-on

b=aspect ratio, sometimes called sidewall profile. This is a PERCENTAGE of the section width. In your case, the sidewall of your 150mm rear tire is an 80-series. That's 80% of the section width measurement for the sidewall, which is 128mm. Lower numbers indicate a lower-profile tire.

c=rim diameter. For making a tire fit just on a wheel, this number is the most vital, followed by section width since if the tire's too wide for the wheel, it'll pinch and wear funny or even fail to seat properly. Aspect ratio will come into play when making the wheel/tire assembly fit your application, since it will affect the overall diameter of the tire and can cause clearance or drive ratio concerns.

Finally, if there's a dash, a "B" or any letter than "R" between number values b and c, the tire is bias-play. If there's an R there, it's a radial.

 

Welcome from Pennsylvania, looks like you got a much better answer than I could provide, best of luck in your search.