Transplanting a large plant, should I completely mix new soil?

What I mean is, I have a large plant about 3-4 feet high which needs more space. I'm going to be adding more soil. It's doing great now except for dying leaves due to lack of root space. I got a bag of soil, couldn't find perlite, or any good compost. Figured I'm going to just add it as is since the plant's pretty strong already and what I started it in is just soil. Should I bother getting the perlite and compost or should I be fine?

Also, I started it in a potting soil, but I ended up buy some peat moss to add for transplant... Will this be a problem? Should I return it for the same mix I was using? I would think it's ok but, jw..

The plant will probably survive no matter what you do.

The real question is - "Do you just want it to survive, or do you want it to reach it's full potential?". If you do everything right, to the MAX, you'll end up with a "Primo" plant. The more things you do "Half-Assed", the more your results will suffer.

Now, with that being said, it probably doesn't make any differance if you're adding nutes independantly from the soil.

Yeah I figured as much, I was just being a little lazy as to adding the extras to the peat moss I bought. The peat moss has a very small NPK base, and I've been adding big bloom. She's nice and big anyway so I doubt she really needs the extra perlite and other additives to the transplant soil. She was in a 3 gallon area already. Now she's about 15, but with no tarp restricting her growth, so if she does outgrow the peat moss, she can go into the natural soil.