In bonsai there seems to be a inflation going on in terms of age. Both people and businesses often claims older than they actually is and it is really hard to tell for sure without damaging the tree. But if you truelly have a 20 year old juniper, I would not recomend doing your first repotting by yourself.
Agree on the suggestion, in general, but just want to point out that "20 years old juniper bonsai" != "20 years juniper plant"
If you get what I mean...
The first is most likely a well-established bonsai, and as such can be very expensive (depending who shaped and maintained it and how), the second just a relatively young plant (and usually rather cheap).
So, if you're in the first situation, definitely get the help of someone with serious experience.
If you're in the second, while getting that same help won't harm and would probably guarantee a faster learning curve, you're also in a situation where you can experiment a bit and learn from your own experience (doesn't mean you don't have to study tho)...