I learned to ride on a 24" uni that my parent’s bought me for Christmas. It has lollipop bearings but was really a good learners uni.
I progressed in skill but do not have much money so I did not get a new uni when I wanted to ride trials and MUni.
This unicycle is totaled in every way possible except for the fact that the hub is in one piece.
I desprately need a new uni because all I have that works now is my giraffe.
I tried a real trials unicycle for the first time last weekend and it was amazing. I felt like I could have jumped over the moon. I could get 12" on my cheap 24" uni and I was probably getting 18" on the trials with almost no effort. I have never even ridden a 20" wheel before that. It was great!
Has anyone else had this problem and wrecked a perfectly good learners uni by taking it too far?
Should I buy a cheap trials uni at least to hold me over?
yep im a perfect example, only it took me 4 unicycles to realize that i should upgrade to a real solid muni.
Started out with a 12" savage, moved to a 16" savage, then 20" savage, and finally a 24" savage. nothing is salvagable in any of these unicycles, except for perhaps the pedals on the 12" 
Disclaimer: i by no means encourage you to buy a savage. however, if you do buy one, be sure to remove the saddle and simply sit on the seat post. this is far more comfortable.
I was in the same position as you about 2 months ago, only i was learning more towards a kh24 than a kh20. I ended up ordering the muni (tho it has yet to arrive). If you know you are hooked on trials, then there is no point getting a cheap uni like a nimbus trials just to “hold you over”. Inevitably, you will brake it and wished you had the kh20 or onza or whatever solid trials you wanted all along.
its up to you, but i dont think many people have regretted buying a kh20.
-grant