I have an 18 RG Special with a 23” tire and 6deg rake that came on the bike when I purchased it. I’m 5’7 and it’s a hoss to get up and make
slow maneuvers versus a 21” wheel on my other bikes. If I simply change to a smaller tire will
it mess with my ride in a negative way? Rides down the road perfectly now
with no wobbles.

 

Im no expert but I believe you will have to adjust the rake and triple tree back to stock. Thats why I stuck with a 21" wheel, I didn't want to mess with changing the triple tree and rake. Nice bike though

 

Did they also extend the suspension for the added rake and wheel size? You may have to deal with that as well.

 

In a word, yes.

But, you will be tipping the bike forward and down. The smaller the front tire diameter, the more the bike will be pitched forward and down.
This will make the bike a bit more tender or twitchy in the handling department. IMO@E, it's not a big deal. I've run fork tubes down several inches in bikes just for that, and never found the bike to become dangerously squirrelly.
It also pitches your seat and handlebars, that can be uncomfortable.
The change in the front frame and engine ground clearance could also become a problem.

So, what size are you actually considering?

 

Keep this in mind. I have a stock trees complete it you want it. I have no use for it . Going price would be one cup of coffee.
You will need to check. They likely added slugs to lengthen the tubes easy to remove.